So here's a question for you:
The key here is that you get the character's entire power set, but you can only use each power once. Do you choose someone with lots and lots of powers, none of which is crazy awesome, or someone who can do only one or two things but at a massive scale? Plus you have to factor in whether you're of a chaotic or orderly bent -- would you use your power(s) for personal gain, or to help others? Here's my take on each of the options, let me know yours in the comments!
- Deadman: Basically you'd be able to become immaterial and take possession of one other person for a few hours, at most a day or two. The question would be who you should take over, and what you would do as them. For instance, you might take over a multi-millionaire, withdraw several million dollars, and stash it in a secure location. When the guy wakes up he won't remember anything from the time you were in charge, meaning no one will know when you go nab the stash.
- Hulk: The choice for those who just want to cut loose and wreak havoc for a few hours. Not my cup of tea, but I know a lot of people like it.
- Jean Grey: A broader power set than the first two on the list, you'd be able to read someone's mind, use TK, and a couple of other juicy bits. To some degree this choice depends on whether you're thinking of early X-Men Jean Grey, or Dark Phoenix-era stuff. Snuffing out suns is heavy sauce, but on the other hand it'd be nice to be able to read your spouse's mind and have a real heart-to-heart conversation about a few things, you know?
- Martian Manhunter: Many useful powers is the main draw of J'Onn J'Onnz. Flying would be an absolute trip, and shape-shifting would have some real possibilities. You could be super-strong for a while, and dematerialize to phase through the occasional wall. You also get telepathy, though not as powerful or far-ranging as Jean Grey's.
- Spiderman: Let's face it, slinging your way through New York looks like awesome fun.
- Superman: The granddaddy of all super-hero power sets, there's not a lot Superman can't do. But it's all physical -- how much good are you going to get out of a blast of super-cold breath?
- Thor: The main power here that appeals to me is storm mastery. Imagine if you could have told Hurricane Katrina -- or now, Gustav -- to turn into a pleasant shower. You'd have an incredible opportunity to save lives. As a bonus, you'd get one free trip to Asgard or some other crazy dimension, which is kind of neat.
- Wolverine: My main thought here is to save his mutant healing factor until you're like ninety years old. Turn it on and voila! You get another 90 years and a fresh young body to boot. Hard to beat that. Alternatively, if you get hit by a bus tomorrow, you've got a "Get out of Death Free" card, which also doesn't suck.