Bellosom: Sludge Bomb, Dazzling Gleam... er.... Drain Punch... to be fair, its Attack is only slightly lower than its Special Attack but... it's not got much in the way of options.
Breloom: Oddly enough, Fighting/Grass is probably its best option. In any event do not make it Poison or Steel if you want to use Poison Heal. It does also get Thunderpunch and Iron Tail. Fighting/Electric Breloom? Only weaknesses are Psychic, Fairy, and Ground at that point, and offensively they have some synergy.
Virizion: Zen Headbutt, Bounce, and Aerial Ace are literally all its non-STAB non-Normal offensive moves. Psychic/Fighting is an OK typing, and Fighting/Flying is actually pretty decent, but Virizion's real problem is having a dumb statblock and there's no major gems in its movepool to save the day,
Tangrowth: Knock Off, Earthquake, Brick Break, Sludge Bomb, Payback, Focus Blast, Poison Jab, and Rock Slide. It is actually slightly higher in Special Attack than Attack, contrary to expectations, but overall Ground/Dark seems one of its best options.
Meganium: It's actually only one point weaker in Attack than Special Attack. Meanwhile, it gets Earthquake, Dragon Tail, Outrage, Iron Tail and... er... Fury Cutter?... no seriously, these physical moves are all its non-Grass attacking moves of any competency. This is somewhat unfortunate, given that every single combination you can produce out of these except Dragon/Steel has at least one double weakness. Alternatively, you can focus on a supporting role and give it typing appropriate to that. Or... just not use it. Too bad.
Sunflora: Sludge Bomb, Curse, interestingly enough Earth Power... well... you could make it Ground/Poison? Or make it part Ghost to put a Curse on people?... Sunflora is bad.
Sceptile: Once Mega Sceptile is a thing, keep in mind the possibility to Water/Flying for Lightningrod abuse. Moves-wise, Sceptile has Pursuit, Night Slash, X-Scissor, Dragon Claw, Earthquake, Brick Break, Acrobatics, Crunch, Rock Slide, Thunderpunch, Outrage, Dragon Pulse, Drain Punch, and Iron Tail. Keep in mind that while Sceptile's Special Attack is higher than its Attack, it's a manageable difference. While a lot of these would be a bad combination with grass-typing, producing one or more double weaknesses (Rock/Grass is the only one of these that doesn't), that's OK because you can just ignore its Grass typing (Maybe use Leaf Blade as a coverage move) and instead make it Dragon/Steel, or Ground/Steel, or Ground/Electric, or Electric/Dark, or... it has a lot of potential!
Shiftry: Shadow Ball, Brick Break, Focus Blast, Rock Slide, X-Scissor, Power Up Punch, Extrasensory, a surprisingly large number of weak, Special wind-themed moves like Ominous Wind, Twister, and Icy Wind... oh, and you could make it Normal typed for STAB Explosion and Fake Out, of course. Normal/Ghost could be pretty good, or Grass/Ghost as a fairly solid side-grade that abandons the Bug double weakness. None of its other possibilities are really exceptional, though.
Cacturne: Poison Jab, Power Up Punch, Belch, Venoshock I guess, Thunderpunch, Superpower, and Drain Punch all stand out. It's too bad it doesn't have anything that synergizes well with Water Absorb. It should probably hold onto its Dark typing to keep STAB on its Sucker Punch, because its speed is bad and its durability is bad too, which makes it difficult to take advantage of its theoretically solid offenses. Poison/Dark? Electric/Dark?...
Cherrim: Dazzling Gleam, and, um, Rollout? I suddenly understand why nobody uses Cherrim. This is awful.
Carnivine: Crunch, Sludge Bomb, Thief, Payback, Knock Off, Bug Bite... er... Wring Out?... Grass/Dark isn't the worst type in the world, but it's doubly weak to Bug and Carnivine isn't getting any pay-off from Levitate. Unless you're in an Inverse Type Control match? Is that even a thing? Nor can it pull off Dark/Poison except as a specifically mixed attacker, which would otherwise be a great combination with its Levitate. You could run it as a more defensive Dark/Poison with no weaknesses, but that's relying on its comparatively weak defensive stats.
Leafeon: Last Resort, Trump Card, Knock Off and... er... Iron Tail?... basically, you could run it as Grass/Normal to power up its unusual and unusually powerful Normal moves... it's... bad.
Shaymin/Shaymin-Sky: Psychic, Dazzling Gleam, Earth Power of course... that's about it. Grass/Ground? You'd still have a double weakness to Ice, but there's some appeal. Or you could go Fairy/Ground and have no double weaknesses.
Serperior: Dragon Tail, Dragon Pulse, Iron Tail, Pursuit, Aqua Tail, Knock Off, Iron Tail and Outrage. Keep in mind Serperior has equal Attack to Special Attack, so a Water/Dragon or a Water/Dark physical build could be good. It's too bad that even once Contrary is released unless it gets new tutor options it's only Special options will be Grass moves and Dragon Pulse.
Simisage: Acrobatics, Shadow Claw, Crunch, Gunk Shot, Iron Tail, Knock Off and Superpower! Dark/Flying, Flying/Fighting, Dark/Poison, Flying/Steel... it's got some nice options. Might salvage it?
Lilligant: Unless you want to count Dream Eater, it's all Grass, all the Grasstime.
Maractus: Sucker Punch, Poison Jab... um... Pin Missile, Aerial Ace... Hyper Voice?... and Drain Punch and Knock Off. It's Special-oriented in stats, but not so strongly that a physical edition couldn't work. Uuuh. Dark/Poison running Sucker Punch, Poison Jab, Drain Punch and a support move?
Gogoat: I've already covered that it can do a mixed attacker Water/Ground with Sap Sipper build, but what are its other options? It's also got Payback, Brick Break, and Wild Charge. (Add Electric straight-up to be pretty flatly improved!) Yup. That's it. I... would probably skip on Gogoat, given its best option is to be a bad Goodra.
Jumpluff: Weirdly enough, it gets Fairy Wind. Honestly, Jumpluff's offensive stats are so bad the thing to do is probably to give it a more defensible type combination like Dark/Ghost, Dark/Poison, Steel/Dragon, whatever, and just ignore its STAB options. Even aside from the fact that it has no strong non-Grass non-Flying attack moves anyway.
Tropius: Earthquake, Steel Wing, Curse and Dragon Dance (!!), Outrage, and Dragon Pulse. It's only slightly better at Special Attack than it is at physical Attack, and meanwhile Swords Dance, Curse, and Dragon Dance are all available vs no Special boosting options. And it gets Leaf Blade which is quite the nice move. Ground/Dragon unfortunately is still doubly weak to Ice, and that's bad for a Pokemon that wants to be able to Harvest-wall, so Dragon/Steel using Earthquake or Leaf Blade for coverage (Or both, ignoring Steel Wing) might be its best bet.
Torterra: Crunch, Superpower, Iron Head, Outrage... run it as Ground/Dark, or Ground/Steel, or Steel/Dragon with Earthquake for coverage, or Ground/Fighting if Superpower works for you, or Steel/Fighting or Dragon/Fighting if you like powerful moves with major disadvantages for your STABs. It's pretty nice actually. Keep Leech Seed!
Abomasnow: Earthquake, Shadow Ball, Brick Break, Focus Blast, Outrage, and Iron Tail. If you don't want to run it as an Ice type, keep in mind you can replace Snow Warning with Soundproof. (You're out of luck if you want to use a non-Ice Mega Abomasnow: suck it up, and use Blizzard) Ice/Dragon Mega Abomasnow could be interesting. Really, just about anything is a better type combination than Grass/Ice. Ice/Ground works OK for Mamoswine, so maybe that?
Whimsicott: Whimsicott isn't a big attacker Pokemon anyway, so the primary concern should be useful defensive typing, such as Steel/Flying, Dark/Poison, etc. Nonetheless, notable moves include Hurricane, Shadow Ball, Knock Off and U-Turn. Keep in mind that it has access to Prankster Cotton Guard, normally held back by the fact that Grass/Fairy is an awful typing. Dark/Poison is only weak to Ground, and most Ground attackers are physically hitting you, so Cotton Guard Dark/Poison might be fairly competent.
Chesnaught: Dragon Claw, Earthquake, Shadow Claw, Payback, Gyro Ball, Stone Edge, and Poison Jab. It's worth noting that Bulletproof can let it get away with stuff like having a Ghost weakness (Most of the time, Ghost=Shadow Ball) or, oddly enough, a Grass weakness. (Many notable Grass moves are affected by Bulletproof) Ultimately most of the moves Bulletproof blocks are sufficiently narrow representatives of their types that otherwise it's probably ill-advised to select typing based on this aspect... but, for instance, making it Ground/Ghost with Earthquake and Shadow Claw, it would only really consistently fear Water, Dark, and Ice moves, and it has the movepool for it. In terms of just movepool though, I'd probably run either Fighting/Ground or Fighting/Steel.
Ferrothorn: It has Curse, Normal typing it for Explosion has appeal, it has Payback which is great for how slow it is, Shadow Claw, Poison Jab, and Knock Off. Plus a surprisingly decent Special movepool it lacks the stats to really take advantage of. The most obvious route is Dark/Poison, which plays nicely with its desire to wall you and kill you with Iron Barbs and possibly a Rocky Helmet too.
Parasect: Cross Poison, Brick Break, Pursuit, Knock Off, and not much else stands out, considering how strongly oriented towards Attack it is. The most obviously possibility is Dark/Poison. It's too bad it doesn't get anything to really abuse its Dry Skin with. Keep in mind it's one of the rare Spore users.
Ludicolo: Oddly enough, it learns all the Elemental Punches, as well as Zen Headbutt, and Drain Punch. It's stronger by a decent margin at Special than Physical, but barring Ice Beam/Blizzard, it has no notable off-type Special moves. There's some weird things you could do with Electric/Fighting physical Ludicolo or the like, basically.
Cradily: Earthquake, Sludge Wave, Sludge Bomb, Curse, and Earth Power. It has Storm Drain, so I would love to recommend a Ground/Rock variant (Either Earthquake/Rock Slide or Earth Power/Ancient Power), but that's still doubly weak to Grass, and Cradily is not a Pokemon that likes switching out too often. A Ground/Fire variant would have no notable STAB available for the Fire end, but you could run it with Hidden Power Fire, Storm Drain, and Earth Power. Cradily is enough of a wall-y Pokemon that this could be acceptable, or even dropping Hidden Power Fire outright.
Wormadam-Grass: An odd case. Keep in mind Wormadam's different forms have different movepools and even different statlines! (Sandy Cloak has the best Attack and Defense, Plant Cloak the best Special Attack and Special Defense, and Trash Cloak is balanced ie crappy) Also, Overcoat blocks damaging weather and powder moves, so typing it Grass, Ground, Steel, or Ice for those reasons is not needed. All three types have Psychic, Shadow Ball, and Sucker Punch as notable off-type-for-them moves. Additionally, Trash Cloak has Gunk Shot and Sandy Cloak has Rock Blast. Otherwise, their differences in movepool are on-STAB moves like Sandy Cloak having Earth Power and Earthquake. Basically, pick Plant Cloak if you want a Special Wormadam, unless you want Earth Power or Flash Cannon unfortunately, pick Sandy Cloak if you want a physical Wormadam, and pick Trash Cloak if you want Steel moves or Gunk Shot.
Rotom-Mow: Keep in mind that the only difference for the purpose of Type Control between the Rotom Formes (Other than the base forme being worst) is that Heat gets Overheat, Fridge gets Blizzard, Mow gets Leaf Storm, Wash gets Hydro Pump, and Fan gets Air Slash. As such, which forme you pick should be about which of those moves you want, unless you want none of them in which case pick your favorite graphic, maybe one of the less-competitively used ones like Fan. Having said all that, an obvious thing to do is to revert to its base forme's Ghost/Electric typing, which it has plenty of STAB options for. Alternatively, it gets Dark Pulse, Signal Beam, and Sucker Punch for notable attacking moves. It could pull off Dark/Bug, or Bug/Electric, or Dark/Electric. Overall I'd probably stick with Electric typing though.
Leavanny: Two of Leavanny's Abilities are notable here: Chlorophyll, and Overcoat. The latter means it can go in Sand or Hail teams just fine, and doesn't need its Grass typing for Spore immunity, while the former makes it a lightning fast physical attacker in Sun. In any event, it gets Shadow Claw, Aerial Ace, Steel Wing, Poison Jab, Knock Off, and a few Special moves that it's probably not worth trying to go for. It could pull off yet another Dark/Poison Pokemon, or Ghost/Dark for more redundant offenses, or you could make it Grass/Poison (Which really is an OK type combination) or Bug/Poison, or Bug/Dark. I dunno if it would be viable, but it is at least possible for it to escape its awful native typing with its double weakness to both Flying and Fire.
Sawsbuck: Megahorn, Wild Charge, Bounce, and several Special moves it lacks the Special Attack to care. Bug/Electric is actually a decent type combination. Or I guess you could go Electric/Flying...
Trevenant: One possibility is to de-Ghost it and run it as a Curse user for the stat gains. It actually has Earthquake, plus less notably Poison Jab. Overall it's probably best off either staying its default combination or going Ghost/Ground, unless you want to use Curse for stat gains in which case Ground/Poison is probably its best bet. Keep in mind that with Harvest+Curse, it can potentially survive for quite a while! Ground/Poison has the additional advantage of letting it ignore Toxic, which is particularly relevant for a Curse-for-stats build that wants to stay in.
Gourgeist: Something to keep in mind is that Gourgeist's Attack goes up with its size, but its Special Attack is not affected. For a Small Gourgeist its Special Attack isn't that much lower than its Attack, actually. Explosion, Gyro Ball, and Flame Charge are the main physical moves that leap out at me, and it's worth noting that Gourgeist's physical Ghost options are terrible: Shadow Sneak or Phantom Force is basically it. Grass/Fire or Grass/Normal could be good for a Small Gourgeist, while a Super Gourgeist is probably best off taking Steel for Gyro Ball, and perhaps holding onto its Ghost typing to avoid problematic double weaknesses. A Special-oriented Gourgeist has access to Psychic, Flamethrower, Fire Blast, Sludge Bomb, Incinerate, Focus Blast, Charge Beam, and Dark Pulse, so a Small Gourgeist running itself as a weak Special sweeper has a lot of options. Unfortunately, Calm Mind or Nasty Plot or whatever to really try to overcome how low its Special Attack actually is.