Hard choice between Applejack and Twilight.
Applejack can be caring, but would know when to kick my flank (I'm assuming I'm getting turned pony, yes?) when I'm not being responsible. Another plus would be having two more awesome siblings, Applebloom, and Big Mac.
Twilight on the other hand/hoof can also be very caring, and she could be very helpful when I have no clue what the hell I'm doing. Shining Armor, and Cadence as siblings would be pretty awesome as well.