I'd have to say time machine... just that knowlege of experiencing/interacting with the past and future is a fantastic thought. What you could learn and your interactions with others.... blargh, it's just too awesome to think about.
At first, I was fully behind the time machine, but being an avid Science and SciFi reader(hell, I'm playing Timeshift currently) I realize how dangerous that could be --I'd be too tempted to fix "bad" things I saw, forgetting that good things might come out later on.
...and while I would definitely like more comment and feedback on DA work (please? ...Pretty Please??)...
I gotta go with Superman's ability to fly. Since childhood I have dreamt of being able to fly like that (still do sometimes). I would never have to deal with traffic (or being late). Vacations would be a breeze because I could go anywhere in seconds and on those "blah" days when the weather sux and I'm bored, I can just grab a coat and my mp3 player and just chill-lax above the clouds.