Derp. Only thought I could choose one
So went swampert. That thing has served me best throughout the years.
But if I could have, I would have gone Charizard, who's served me as the rock to ground my Blue/FireRed teams on. That thing... oh, Charizard. Remember the days when submission was a great move because Charizard in the anime took out a magmar with it? aaaaah....
Gen 2: Typhlosion. That thing was amazing! (gen 2, that is. Not HeartGold.) I probably would have gone for Feraligatr, except that thing didn't make it to 100 like mine did and it hadn't got ditched at goldenrod
Gen 4: Torterra. Chose him as my first starter and good grief! He's a beast! A tank! Pure good fun for ingame! Close tie with the other two though. They're all quite balanced (ingame, not for realz.... poor torterra....)
Gen 5: Samurott, but I haven't played with any of the others, so it's hard to say. Samurott's a great set-up sweeper, but so is Serperior.