Tie between Twilight, Scootaloo, and (big surprise coming!) a Changeling. I'd like to go on adventures as one of the 3, really.
Twilight has a lot of things going for her, Mane 6 being only one of them. Being the sister/disciple of the 3 princesses also sounds like a rather nice perk, in my opinion. She also has the instant win button, so I could be mischievous and live to see another day.
Scoots would be interesting because I do believe that high-speed ground travel AND having the adorability that rivals that of a rabbit while being a part of a club'd be pretty cool. It'd also be very nice to be good pals with the other crusaders, as they also have their own brand of ideas.
Changeling.... Favorite species and stuff... however, I think people will disagree with my thoughts, but here goes. Yes, there is the obvious "OMG! It looks like a monster! KILL IT!" mindset of some, if not many, ponies. However, I do think that some would be accomodating, as some ponies actually like them and when it comes to what factors are canon or not of them is about as plentiful as the choices of candy you can buy for 5 cents. I'd like to explore the world and just become anypony I wish and just try to enjoy life
Just my two bits..