I'll do them by their first Forms:
Gen I: Blastoise(Blastiose can handle well: Brock, Koga(due to Ground Type TMs available such as Earthquake, Sabrina(due to Bite), Blaine and Giovanni due to super effective STAB, and Ice for Giovanni as well)
Gen 2: Totodile
Gen 3: Mudkip
Gen 4: Chimchar
Gen 5: Oshawott(Good with Elenas Zebstrika with Dig, good with Clay because of STAB, Good with Skyla because of Ice Moves, and the same is said for Drayden.)
Gen 6: Fennekin(Super Effective STABs on Viola's Lead, Korrina's team, Ramos's Team, Clemont's Team, and part of Valerie's team, and Fire Stab on Wulfric.
Gen 7: Popplio(Good on the Olivia, Nanu, and Hapu Grand Trials, the Fire Trial and Dragon Trial, and if you overlevel it, potentially good with a Shadowball on the Mimikyu Trial. Good on all four E4s, with Shadow Ball for the Ghost Lady, also good on some of Kukui's team members, such as Lycanrock and Braviary, as well as even the Decidueye with Acrobatics TM.)
Gen 8: Both: Sobble(Good on the Grass Gym, Fighting Gym, if Acrobatics is available,I'm not sure since I've never played, , Ghost Gym with Dark Moves, Fairy Gym, with Mud on most of 'em,ss, the Rock Gym, some of the Dark Gym with Bug Moves, and the Dragon Gym, as well as Leo's Charizard.)
Hopefully I did well on that, I've never played SWSH, so I'm not sure if I misput something(ie, having a TM before it was available.)
Sources: Serebii, trusting in the comments, a bit of Experience(I always pick Mudkip for Gen 3 games.) and the Starter Pages.