Slowpoke or Haunter.
Haunter has a special place, because I used him to cheat through the Pokemon leagues because I hate grinding. (level 30 Haunter, 5 stallers, buy 99 Revives, use Curse. Good stuff.) He's also purple and cool-looking.
Slowpoke...he's pink and derpy. He has more personality than most others combined. Also he says "...Yawn?" in the gameboy games whereas most other Pokemon just say (a stupid variant of) their own name.
And I have had Slowpoke as an avatar for about 2800 posts. So yeah.