Guess three times...
Eevee, He's cute, and you can actually use him in VS battles (Evolved or not)
I like all the Eevee evolutions and I'm actually working on breeding an Eevee evolution team (All 5 evolutions plus Eevee himself)
Lugia, Latias/os, or Vulpix comes second but are just as fun to play with...
I actually prefer the Lati duo to Mewtwo, but that's just my preference (Dragon/psychic is better in some fights)