The Good
BW2 Lucario. So I followed the official guide for Gym 1, which advised me to raise Riolu to level 15 and get Force Palm, which as it turned out OHKOed both of Cheren's Pokemon. I then let the rest of my team catch up, put it back in ready for Gym 2 and it evolved early.
Lucario is nothing short of a beast. Obviously insane offenses earlygame, but also takes earlygame attacks like a champ. Immune to sandstorm and poison. And he doesn't need to have supereffective moves to just carve up standard trainers. I ended up giving him Leftovers because that's how well he was just brushing off everything. What a god. He would fall off lategame if it wasn't for that insane movepool...
XY Farfetched
You want a sign of how easy XY was? In the earlygame there's a trade for this monstrosity. My brother took it, and only went and used it for the rest of the game. Since it was a traded Pokemon, it gained EXP at a faster rate. Which, thanks to the reworked EXP Share meant it was 10+ levels above the rest of the party. And therefore at least 20 levels above everything it was fighting. It unironically hit Level 100 while he was fighting the Elite Four (level 60s). Just. What. A. Beast.
DP Infernape
We all know Gen 4 Infernape was a beast. But 9 year old me did not. He just liked the Fire type starter, and didn't have any concept of training up other Pokemon. So my Infernape, aided by a supply of sacs and Revives, pretty much soloed my first Pokemon game. The only other mons that did any real work were my Crobat that I'd caught at a high level and made into my Flyer, and Dialga, who chipped in some stuff using only the 4 moves I caught it with. I got stuck on Bertha for a while (I thought Close Combat as bad, so had to go back and learn it again...), but once I made it through that... Yikes. Many Roserade and Lucario were slaughtered.
The Bad
The entire Pikachu line in literally any game
I don't actually dislike Pikachu for its popularity. I don't dislike it for now competitively useless it is. I hate it because it is garbage in game. In any game where you have the misfortune of adding this rodent to your team, you will be left with a midspeed frail thing that only beats things it is Super Effective against. Which is two types. And god forbid you have to catch it as Pichu. That thing has to be carried so hard it loses to anything that isn't Water/Flying. And even then it might be outsped and OHKOed. Then finally, when that trauma is over and you finally expend a precious Thunderstone... It still has the exact same issues. Totally reliant on TMs for coverage. Midspeed not outstanding. Frail. Doesn't hit hard enough. Next.
DP Luxray
As much as I love this thing's design (who doesn't love Shinx), it gets a physical movepool with Spark (better than Thunder Fang as it is same power with more accuracy and secondary chance) and Crunch as its best moves. I got lucky and mine had +SpA so I was able to teach it Thunder to actually hurt things with. But yeah, this thing is unfortunately terrible. Especially if you're stuck with Rivalry rather than Intimidate. Oh and despite it being on the second route, it sucks in the first two Gyms yay.
BW2 Zorua/Zoroark
Yeah, this thing is kinda bad unless you go hunting TMs, whereupon it becomes bearable in the lategame. Because when you get it, for coverage it's one of the dozens of mons at this point who learn Dig and it gets Incinerate by TM. That's it. Hope Foul Play works because not only is this thing frail AF, it hits like a wet paper bag too. Oh and Illusion is just a gimmick 90% of the time. Best of luck.
The Ugly Ducklings
XY Amaura
Ok XY was an easy game, but this was nothing less than a joy to use. Refrigerate was so much fun to play with, and it gives you access to decent moves not called Ice Beam. What did kinda spoil Amaura though, was its evo condition. Iirc, you have to level it up to or above level 40 at night. It was so obscure for me that every other mon in my party was fully evolved. I tried everything from stones to levelling it up in the rain, until finally I found it out by a last-resort Google search. Then it was back to UNBRIDLED JOY of a decent Ice-type.
DP Drapion
I picked up this thing specifically because I loved CritDra, only to discover to my disappointment that guess what, Scope Lens is postgame, Cross Poison is learnt at Level 58 and Night Slash is an Egg move until BW2. However, to my pleasant suprise, Drapion turned out to be a beast anyway. It has great defensive capabilites, comes with elemental fangs, and Dark is a lean, mean attacking type with Crunch. Plus Sniper Pin Missile Skorupi was hilariously RNG-heavy to the point that it could just occasionally wipe something off the face of the map it really shouldn't have lol.