Back when I was a lot younger, the only one I'd use was Bulbasaur. Bulbasaur destroyed the early game! It could single-handedly take out Brock, Misty, Lt. Surge...
Now I've branched out a lot more and I find that I like Charmander the most, as I try to make the games as challenging as possible now. Charizard seems to be the strongest of the three, even if it has the fewest favorable matchups in Kanto.
I've never been too fond of Squirtle, but that's not to say I don't like it. I've gotten it to Wartortle a few times (never Blastoise, sad to say) and it's been fun using it, but it doesn't stick out as particularly great.