Gen 1: Nidoking. Just because it's cool.
Gen 2: Either quagsire (becuz I only used clod) or octillery (it's a cool looking octopus Mon)
Gen 3: Ninjask and Shedinja (Package deal, mostly just want to try trolling with the latter of speed boost go brr with the former), also maybe aggron as I've never actually used one.
Gen 4: Lucario (weirdly never ever used one), Gastrodon (the cry, enough said),
Gen 5:Bravairy. Rufflet was on both my swsh and SV teams and I've never evolved it. Still want to use this at some point, be it normal of hisuan, becuz I heard about it from my younger brother talking about it in the adventures manga, and also it looks cool. Also maybe leavanny (becuz sewaddles dub voice is adorbs) or amoongus (to do something other than spore, rage powder, pollen puff ...)
Gen 6: Avalugg (didn't really use it for my Kalos only scarlet run) or dedenne
Gen 7: Oranguru/Passimian (I've not used either at any point) or Mimikyu (becuz it's Mimikyu)
Gen 8: Obstagoon or Perserker (my favs of the galar evos), if not Toxtricity or grimmsnarl
Gen 9: Kilowattrel (cool bird) or Mabostiff (so I can remember arvens tale)