If I'm going to be selfish, I'd say to be fantastically creative at music and guitar. From my few experiences of playing live, it's pretty fucking awesome, and I honestly couldn't see myself getting bored of it. To be in a successful rock band, to be playing arena/stadium tours, huge festivals... I'd basically be doing something I'd pay a lot of money to be able to do, only I'd be actually be paid quite a lot of money to do it instead.
Sure, doing something worthy of a Nobel prize would be nice and fulfilling, but I'd imagine that the self-satisfaction would sort of wear off after a while, and unless you've created something directly useful and patentable (which is fairly rare I'd expect) then you're not going to have a huge amount to show for it financially either.