I have two separate teams, one that spans all generations and another that's exclusive to that generation. So I'll pick up whatever ones I like most into my Gen VI-exclusive team and add one in particular to my all-star team, replacing either Seviper or Gardevoir to make it truly span all generations.
As far as the Gen VI-exclusive team, I'm guessing it's gonna be a mixture of Bug, Poison, Steel, and Fire types, for some reason I gravitate towards those.
I only use Pokemon that can be legitimately caught in a region for my team. This means that I never trade pokemon to use on my team (I do trade them for possession, though). Which also means that use mostly Pokemon from that region. Though, with the exception of Black and White (and Gen 1), I always use at least one Pokemon from earlier generations. For Gen VI though, because of the 3D effect, I might have to check out some of the older pokemon as well as some of the Gen VI exclusives (I will obviously have my Gen VI starter... Probably Chespin as it's looking right now, because Chespin is adorable!)