My hands-down favorite since gen one was always Gloom. I find the whole notion of a horrible smelling, slobbering, unhappy looking flower creature just incredibly neat and the whole design appealing. Vileplume is based one on of my favorite plants (the Rafflesia or "corpse flower") but Gloom has that extra "creepiness" to it that I love.
Nothing ever came close to surpassing my love of Gloom until gen V brought us...
Garbodor! I'm a life-long fan of monsters, which is what attracted me to Pokemon in the first place, but my favorite monsters are always, well, the
monstrous ones, and what's more monstrous than a rotting, shapeless, bug-eyed, jagged-toothed pile of living garbage? I happen to draw monsters as a major hobby, and have several of my own trash-blob creatures, including one I drew years ago that I just happened to call "garboid," you can see it
here. I've got other trash monsters
here and
here (that one's a garbage bag rat, if you can't tell)...so yeah, Garbodor is just the sort of poke I'd have designed myself.