1)Ampharos (and its evolution family): I don't like slow Pokemon in general, and the thought of a slow Electric-type Pokemon always disgusted me. Plus it's bulk and has Static, so it's frustrating to deal with.
2)Gyarados (especially before IV gen): Water-type Pokemon are intended to be Hm slaves, so they should at least be able to make some of use of them; then this thing shows up with it's garbage Special Attack, and it doesn't learn Flying-type moves, and gets Dark instead of Dragon upon Mega Evolution.
3)Simisear: a fat monkey with an irritating face.
4)Dartix: it looks effeminate to me.
5)Serperior: you choose Snivy as your starter because it will look cool when it evolves, but you get a Contrary-less presumptuous snake.
Dishonorable mentions: Feraligatr (same as Gyarados), Bibarel and Palpitoad (dumb faces), Steelix (Pokemon Ranger...), Ludicolo (dumb AND gave me troubles in Pokemon Ranger), Bouffalant (it's Tauros X Afro Rufy), Kyurem (it's asymmetric!)
Special dishonorable mention: Ash's Pikachu: he defeated a Onix, a Dragonite, travelled through many regions only to lose to a Panpour because he slipped on the mud; he also lost to Trip's Snivy. I hate him (I also hate Ash)