I'm really surprised Fire is losing in the poll, it always seems to be the most popular of the three... or so I thought.
Anyway, Grass all the way yo. Five great lines all with different potential. Fire is all mixed offence and speed (barring Emboar on the speed front), while Water is pretty much all bulky offence. Grass seems to have a great mix. Helps that I prefer all their designs aswell :P