I've cycled through most of them, buuut...

Gen I: Charmander, almost always.

Gen II: I LOVE THEM ALL SO MUCH BUT CYNDAQUIL IS MY FAVORITE POKEMON EVERRR

Gen III: Torchic. Sensing a theme here.

Gen IV: C-C-C-COMBO BREAKER. Turtwig.

Gen V: Aaaand back to fire. Tepig.

As for Gen 6? Probaby the fire one. But that frog is hella cute.

BONUS: FAVORITE EEVEELUTION = LEAFEON ALL DAY ERRY DAY