Psyduck, Golduck (all three games)
Availability: For some reason, Psyduck's availabilty is listed as mid-game on the current guide. This is completely false. Psyduck can be caught in Ravaged Path before the first (DP) second (Pt) rival battle. Psyduck starts evolving into Golduck at level 33, which is kind of late.
Stats: Golduck has good all-around stats, although none of them are particularly high. Its 80/78/80 bulk means it can take hits and doesn't need to be revived constantly. It has 95 special attack and 85 speed, making it relatively good for in-game use.
Typing: Psyduck and Golduck benefit quite a bit from their pure water typing. They only have two weaknesses, and they get STAB from surf, making surf even more powerful than it already is.
Movepool: The only good moves in Psyduck's and Golduck's level up movepool are water gun, water pulse, and confusion. Because they eventually become obsolete when the HM03 surf is obtained, and because Golduck only needs surf to win most battles, it's probably best to make Golduck an HM slave. In addition to surf, Golduck can learn rock smash, strength, rock climb, and waterfall. While it can learn confusion by level up, Golduck already beats most poison Pokemon by spamming STAB attacks and loses to Roselia and Roserade even with confusion.
Major Battles: These are the notable opponents that Psyduck (or Golduck) can defeat without overleveling or using TMs. As you can see, its usefulness drops at around Hearthome City and then it can't do much until it evolves, but it's pretty good for the rest of the playthrough.
Barry (Route 203): Chimchar
Roark: Geodude, Onix, Cranidos
Mars (Valley Windworks): Zubat
Jupiter (Eterna City): Zubat
Fantina (Pt): Duskull, Haunter
Barry (Hearthome City): Starly, Buizel, Ponyta, Monferno
Barry (Pastoria City): Buizel, Ponyta, Monferno
Crasher Wake: Quagsire
Cyrus (Celestic Town): Sneasel, Golbat, Murkrow
Fantina (DP): Drifblim, Gengar
Barry (Canalave City): Torterra, Monferno, Infernape, Ponyta, Rapidash, Staravia, Staraptor, Buizel, Floatzel
Byron: Bronzor, Steelix, Bastiodon
Saturn (Lake Valor): Golbat, Bronzor, Toxicroak
Mars (Lake Verity): Golbat, Bronzor, Purugly
Candice: Medicham, Sneasel, Piloswine
Cyrus (Veilstone City): Sneasel, Golbat, Crobat, Murkrow, Honchkrow
Saturn (Veilstone City): Golbat, Bronzor, Toxicroak
Mars and Jupiter: both Bronzors, both Golbats, Purugly
Cyrus (Spear Pillar): Honchkrow, Crobat
Cyrus (Distortion World): Houndoom, Honchkrow, Crobat
Volkner: Ambipom
Barry (Pokemon League): Staraptor, Infernape, Rapidash
Aaron: Yanmega, Scizor, Beautifly, Dustox Vespiquen, Drapion
Bertha: Gliscor, Rhyperior, Quagsire, Whiscash, Sudowoodo, Golem, Hippowdon
Flint: Houndoom, Flareon, Magmortar, Steelix, Lopunny, Rapidash, Drifblim, Infernape
Lucian: Girafarig, Medicham, Bronzong
Cynthia: Gastrodon, Spiritomb, Lucario
Additional Comments: Psyduck and Golduck are exceptionally typical Pokemon (water type, 500 base stat total, no amazing abilities, no interesting moves, medium fast growth rate, level evolution), so I don't really have anything to say here. Due to Psyduck's very early availabiltiy, good overall stats, strong STAB in surf, and my lack of anything to say here, I nominate
Psyduck for B tier in all three games.