I severely dislike Pikachu although I like using Pichu and Raichu myself. Pikachu besides being the mascot, doesn't appeal to me, but Plusle and Minun don't even have a fifth-generation strategy on smogon, which makes them seem useless to me, though they are cute.
Luvdisc is really annoying, Zubat makes my trip through any cave miserable, and on that note, Woobat does not seem any better. Roggenrola with Sturdy I would appreciate more if it didn't take so long to kill them, but I don't consider them useless because Gigalith is awesome. Magikarp at least will evolve into Gyarados, and Feebas into Milotic. Why won't Luvdisc evolve into anything better?
Unown... I spent so long trying to find ! and ? Unowns to get a full set in Leaf Green and I never did and Hidden Power being there only move makes them exceedingly useless, I mean, at least you can put TMs on Delibird, though that's on my list too.