In no particular order:
Charizard. 'Cause memories.
Lucario. He's just a cool dude, y'know?
Quagsire. That face is the greatest face in all of Pokemon.
Golurk. In case you didn't know, this Ground/Ghost type can learn Fly. Yup, Fly. You can ride on the back of a flying undead Transformer. If that isn't cool, nothing is.
Totodile. Dem Pokemon Mystery Dungeon: Explorers of Time feels.
Blaziken. Again, memories.
Zapdos. My first level 100. Drill Pecking the crap out of Bruno about a million times is oh so satisfying.
Spiritomb. This guy is 108 ghosts stuffed into the cutest little rock. And he will pound yo' face in.
Dialga. Time travel is cool, giant metal dragons are cool, and MEMORIES (Also Primal Dialga. Primal Dialga is the bomb.)
Shedinja. There's nothing better than being able to do ABSOLUTELY NOTHING to this guy. Of course, that's rarely the case, but still.
If you're forcing me to assemble a team of 6, I'll grab Blaziken, Lucario, Zapdos, Dialga, Golurk and Quagsire. Maybe swapping Dialga for Golurk.
Honorable mentions to Phantump for being an actual child, Grovyle because PMD: EoT, Skymin because Skymin is cool, Giritina because Distortion World (And, y'know, being a GHOST DRAGON), Accelgor for looking pretty sweet, Tyranitar because it's probably actually like above Shedinja but Shedinja needs some love, Mewtwo for being a total beast, Swirlix as runner-up for greatest face, and Darmanitan because he's just a cool guy. That's all I got.