Batman’s a little too psychotic for me, although nothing would beat constantly being able to save the JLA’s bacon…
Flash, to me, has always just been cool, so that’s a possibility.
The Doctor from The Authority. Of course, with all he’s been through… Hawksmoore from the same place might be interresting. I dig his powers a lot.
Villain? Ra’s Al Ghul. Hands down.
I’d pick Magneto, at least from the days when I used to read Marvel back in the early 80’s through early 90’s. He can use his Master of Magnetism ™ powers to do all sorts of nifty things, and if I’m gonna have superpowers, darnit, I want powerful superpowers! 
Too many to choose from!
Timberwolf or Karate Kid (Legion of Super Heroes)
Booster Gold
Adam Strange
Adam Warlock
Captain Mar-Vell
Captain Atom
But at the top of the list: GROO
When I was 6, Popeye was my hero. I was a big Superman fan. I even sent in for membership in the Supermen Of America. There was a short-lived comic book hero, though, who was cooler yet. Brain Boy had a lot of appeal for smart-but-wimpy kids like me. I envisioned levitating myself above the throng in the school halls, so I could get to the next class on time.
It’s hard to decide. Right at the moment, I think I’d go for Mister Fantastic. The stretching stuff’s neat, but the best thing about him is he is smart. Really, really smart. God-like intelligent, actually.
The best example I can remember is during the original Secret Wars, when the Molecule Man had dropped a mountain range on the heroes. The Hulk, Thing and Iron Man were able to brace up the roof of a small cave, but they were trapped and going to be crushed in a matter of minutes.
Reed starts taking apart the power modules of Iron Man’s armor (remember, James Rhoades was in the armor at the time, so the inventor of this very complex, exceedingly high-tech weapon system was not on the Secret Wars planet). Reed has never seen how the armor is put together before, but he is able to gerry-rig a bunch of spare components from Hawkeye’s tech arrows and Spider-Man’s web shooters into a system allowing the Human Torch and Captain Marvel to pour their considerable energies into the armor.
Iron Man’s repulsor rays are now able to blow away a couple of miles of granite between the heroes and daylight with one blast. And while he was tinkering, Reed even had enough spare mental clock cycles left over to taunt the Hulk to make him angry, which made him strong enough to hold up both his and Iron Man’s share of the mountain range.
This all took no more than 10 mintutes, though anyone else would have had to analyse the armor for a month before he’d have understood it well enough to modify it at all, let alone make it a thousand times more powerful.
I would really enjoy being that smart.
Problem with many superheroes/villains is they get very little time to themselves. Since wolverine has been taken, I think it would be good to be cat woman, as you don’t have to spend all your time trying to rule or save the earth, and get to wear a very kinky costume.
My initial pick would be The Rocketeer, the version from the movie. Hey, Jennifer Connelly would be in love with me! 
One of the mentalists. I don’t even care which; for me, telepathy and levitation are where it’s at.
Or latter-day Swamp Thing, I guess. Being in tune with nature, wrapping whole cities in verdant destruction when they mess with my wife…I could get used to that.
By the way, Kythereia, you didn’t say why you want to be an acrobatic, not-equipped-with-a-full-deck clown groupie. Care to elaborate?
Hey, if you went with the comic version, you get a woman clearly based on Betty Page at the beginning of her pin-up career. So you can’t lose either way.
Had a chance to think about villains, and I’ve narrowed it down to two.
My first choice would be Thanos. The dude winds up with nigh-infinite powers on a weekly basis. You want a villain with power, Thanos is your man/demi god.
My second choice would be Dr. Doom. Yeah, there’s the scarring and the craptastic name. But at the end of the day, you have your own country as a consolation prize if you lose the world conquest thing. Not bad at all.
Okay, I now want to see a Swampy / John Hawskmoore battle royale.
Like the poster says, “No matter who wins, we lose.”
It’s over-rated. 
His power level varied according to writer, and was effected by certain events (Like when Mutant Alpha regressed him to infancy).
The following are standard-
Levitate by pushing off the earth’s magnetic field.
Fine control-the ability to pick locks, and some control over electrical devices (essentially having a built-in remote control)
The abiility to pick up truly massive pieces of metal (varies. He once lifted an abandoned nuclear sub from the ocean floor and held it a few hundred feet in the air while he removed the nuclear missiles)
The ability to reshape metal.
Sine the mid 80s or so Magneto has been able to use his electromagnetic powers to stop the human heart, cause a persistant vegitative state through massive but deliberately nonlethal brain damage, and save lives by acting as living defibrilator paddles. He can also control blood due to its high iron content.
Still, my favorite demonstration was during Secret Wars. The Wasp was upset with what the humidity was doing to her hair. Through intense concentration, Magneto was able concentrate enough of the trace metalic particles in the atmosphere to make her a comb.
Ambush Bug because I think the knowledge that I’m actually in a comic book would make all that hero angst a bit more bearable.
I have always wanted to be The Flash. Something about that high metabolism, and being to eat junk food all day, and I guess being real fast would be cool too.
Villain: Solomon Grundy, natch. Not the new, super-powerful special-needs zombie version of him, but the old Superfriends version, where he was a good bit less powerful but a little bit more literate. He was just a bad-ass, all dead and cajun and shit.
Hero: Green Lantern. It really is the coolest superpower, a ring that’s only limited by what you can imagine. Only problem with it is that if I got a version where you had to recharge it every day (some of the GL’s don’t have to do that anymore, right?), I’d probably end up forgetting all the time, and then the ring would just end up in a drawer somewhere next to my class ring and my watch which I never bothered getting new batteries for and that earring I wore for about a week in college but stopped because it “made me look gay.”
Hero, or at least on the side of good:
The Great Will of the Macrocosm, from Excel Saga, also known as Will-chan. Nothing beats being the Deus Ex Machina. But I’d probably dump poor Pedro, though, and start a harem. coughs
Villanous, or at least has highly questionable motives:
Xelloss, Slayers Next. Manipulative, sneaky, and gets away with everything. (Been bringing him up often lately since I’m writing a fanfic with him in it and I’m having so much fun imagining his dialogue. :D)
If it could be me with the powers, I would pick The Specter.
No to the cosmic retribution/powers vary depending on God’s will/the whole dying thing.
Yes to the abilities to fly, go through walls, appear and disappear at will and melt anyone with a thought.
As for bad guys, I would be Ultraman from the Crime Syndicate. Vaporizing anyone who spoke out against me and doing Superwoman whenever I felt like it.
I think it’d be cool to be one of the Power Rangers. You’re basically indestructable, you automatically know a bunch of martial arts, all of your enemies are incompotent, you get giant robots and cool weapons to play around with, and the only time your enemies are ever more powerful than you are, it’s only temporary and leads to you getting even more robots.