I can't really say I hate any single pokémon. I find some a little silly but over time they have grown on me. I used to think Trubbish was silly and Garbordor was even worse. After thinking about it, that little guy is way better than its cousins Grimer and Muk. It goes from a trash bag to a landfill. I think it's actually creative.
Well, several people have already mentioned Vanillite, which makes me happy, because I think it's adorable! Now, I'll be honest, when I first saw it I thought "An ice cream cone?! That's stupid!" But the little tyke grew on me, and I decided that the design was rather creative, a lot better than many first-gen Pokemon.
Also, RATTATA. It wouldn't normally be one of my favorites, but it was the first Pokemon I ever drew when I was a little kid, and Pokemon was what really got me interested in drawing in the first place, so it has a special place in my heart. I even trained the first one I caught in SoulSilver to level 100. <3