Wow, it's really difficult to pick.
Normal: Porygon-Z. I really love its animation and it's the only Normal type that I've used regularly in competitive play.
Flying: Staraptor, definitely. I've never gone a DPPt run without getting one because its power is so consistent through the game.
Grass: Lilligant. It's one of my favorite Pokemon of all time and I used to do decently when I battled with it.
Fire: Infernape. Infernape Infernape Infernape Infernape Infernape.
Water: Feraligatr, which is a close second for my favorite final evo of a starter behind Infernape Infernape Infernape.
Electric: Electivire or Rotom. We'll go with Electivire because I like to get physical.
Ice: Vanilluxe. [laughs hollowly]
Bug: Heracross and this will never change no matter how many Bug types are introduced.
Rock: Cradily, oddly enough. I used to love using Cradily.
Ground: Hippowdon. We made many gen 4 memories together.
Steel: Skarmory, I guess. I like using them in gen 3 but I've never used one in any other game.
Fighting: Infernape Infernape Infernape Inferna If we're counting secondary types, Gallade.
Poison: I like Skuntank a lot but Weezing is better.
Dark: Murkrow, both in-game and competitively.
Psychic: Mesprit. It's pretty flexible, but it doesn't specialize in offense or defense and that hurts it.
Ghost: Mismagius, but I never really learned how to use one effectively.
Dragon: No clue, really. I've probably used Flygon a lot more than any other, but I liked using Dragonite more.