Hero:
Orion. No contest; the Dog of War, especially as portrayed by Walt Simonson in the eponymous series of recent years, has it all -- vast power, nobility, tragic flaws and the determination to overcome them that makes a hero. And the Astro-Glider is cool.
Which kind of makes the villain choice inevitable:
Darkseid! Of course, DC in its madness wants to downgrade the "Big D" to a second-string Superman villain, which is... well, mad. Fond as I am of Superman, the New Gods should be a step above (wot, only one? :rolleyes: ) the usual hero/villain squabbles.
Other heroes I particularly like...
DC: Green Lantern (Alan, Hal or Kyle; the others mostly leave me cold); the Flash (Jay, Barry or Wally); the original Firestorm, not the dross they're currently publishing by that name.
Marvel: I'm not a Spidey or FF fan, really, though I have read them, and I gave up on the X-Men years ago. I go for Thor and Iron Man -- who make a great team IMO. Sort of the Marvel equivalent of the World's Finest team (that's Superman and Batman, if you didn't know), or maybe Flash and GL.
Other: E-Man and Nova! Brilliant comedy, excellent satire, and probably the sexiest woman in comics.

GrimJack was a favourite in all of his various incarnations (literally

). And then there's...
Oh, that'll do. It's obvious that I read too many comics -- or, rather, that I've read an awful lot over the years. These days, with DC and Marvel firmly in the let's-ruin-all-our-characters-so-that-brain-dead-kids-will-think-they're-cool mindset, I'm slowly slipping into humour titles;
PS 238 is brilliant, and
Nodwick has achieved the impossible (or at least very hard) by hooking a person who doesn't much like mediaeval fantasy...
Phil, who'd add Captain Carrot and Sgt Angua -- not to mention Detritus and Commander Vimes -- to the list, but they don't qualify as super-heroes... or do they?