To me, the seemingly obvious answer is Champions / Hero System, but it's not as clear cut as all that. While I know that Champions and the Hero System have a bit of a reputation for complexity, the core mechanics (3d6 roll under) is pretty simple. The heavy lifting comes with character creation, which is a bit of a double edged sword. While Champions lets you create just about anything you can conceive of, it can get a bit overwhelming if you don't already have a concept. Building the powers can be a bit of a turn-off to some, but once the characters are done, the game flows pretty smoothly. Combats do tend to take a bit of time, but that's pretty much in keeping with comic books where you generally only see one big fight per issue.
Mutants and Masterminds of course is in the mix. I love the DC Universe supplements which means that I'd never run out of ready-made antagonists, but it doesn't seem to be that much lighter then Champions. Character creation still seems to be the stumbling block for new players, but man is it a beautiful book.
GURPS is that game that I've always wanted to love, but just don't quite. Sort of like Champions without all the stats, which is probably why I'm not that fond of it. Had I come to GURPS before I'd ever played The Hero System, I'm guessing that this would have been my go-to game, but it didn't happen that way. Plus, I'd need to tote around at least 3 books just to do character creation.
So maybe Savage Worlds? It's a great system for pulp games, and would probably be awesome for a costumed vigilantes fighting Nazis type game, but I'm not sold on it for straight up Superheroes. I need to do a d6 only have for Savage worlds, as I'm just a fan of using d-sixes and.
Finally, we come to Icons. I picked up the Assembled edition last winter and it really seems to scratch that more rules light while still doing supers itch that I seem to have. I like the idea of random superhero creation, and it uses all six sided dice.
In the end, this doesn't really go anywhere. I doubt that I'll run anything soon, but it may e a fun diversion for the times when all of our regular group can't get together. I think that either Champions or Icons would win out, but who knows?