Grass: Venusaur - truth betold, I've never had a Grass Pokemon in my team after RBY, and I chose it in the first place because I like the name Bulbasaur. Still, I can't remember being disappointed with good ol' Veny when battling all through Kanto
Fire: Charizard - sentimental reasons probably overshadow my judgment in this case, but the heart wants what the heart wants! It can fly, and it has FIRE COMING OUT OF ITS TAIL and it's almost dinosaur-like in appearance. Normally I'm satisfied with most of the fire starters, but Charizard holds a special candle to my heart.
Water: Swampert - mainly due to its awesome type-set. Truth betold, I can't remember a lot about it, but if I say Blastoise it'll just appear that I have a massive fanboygasm over Gen I, Feraligatr's horrible name is an instant rejection by me, the current Samurott on my Black team is getting replaced the instant I get a Lapras and I didn't play Diamond/Pearl, so have no idea about the merits of Empoleon.