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