I'd choose all but one of the transformational masks (though the one I don't choose would still be my next choice):
-Deku Mask because I could easily reach high places that I couldn't go normally (and maybe begin to conquer my insane fear of heights at the same time). Plus, I'd rock at a bubble-blowing contest.
-Goron Mask because I wouldn't even have to worry about buying a car, gas, insurance, etc. because I could just roll around really fast. I'd just have to buy some green potion (maybe it'd be called "Green Bull" instead of Red Bull).
-Zora Mask because I can't swim for very long without tiring out, and I could also use it to save someone should they start to drown.
-Fierce Deity Mask because...well you know...
The Giant's Mask would also be good for obvious reasons. 8) It would probably get annoying to have all that attention, though, so that's why I didn't choose this before any of the other transformational masks.