Well I haven't been playing very long, but if I had to choose 1 pokemon that is my favorite, I'd have to choose Venusaur!
This may sound kind of cheesy but I can't help but love my very first Pokemon (even though that's bulbasaur)! It's been with me through the whole game and has never let me down in a battle <3 I heard his type set-up isn't too bad (Grass/Poison). My Venusaur and I have taken down every gym leader with just the two of us. I trained other Pokemon but for Gym Leader battles I try to just you Venusaur and we show all those pesky gym leaders whose boss! =3