I would probably be a Pikachu-I ALWAYS get him in mystery dungeon, I'm very jumpy(adhd), love pranking people, and just wanna have fun. As for legendaries? KYUREM! EPIC, ICE-ARMORED, DRAGON! He can freeze a whole forest(see Black/White),learn lots of different moves, and looks really cool, Kyurem is the first Ice/Dragon, and he has a cool name.
Well, I don't know about personality-wise buuut...
If I could be one I would have to choose from Aron, Cyndaquil, or Lucario simply because they're awesome and become awesome. Cept for Cyndaquil, I like him better than his evolved forms.
For a legendary....well, either Zekrom because he's my best, or Darkrai because I'd love to have power over shadows/darkness/nightmares. Neither are my favorite legendary. I don't have a favorite legendary....at least not that I remember or know of.