The case for Mutants and Mastermind.
I can only speak for 2nd edition, but to me this is the best superhero game I played.
Mutants and Mastermind works wonderfully for a game where people are just over the top competent. Not only superheroes but just amazing characters beyond humans capabilities. The d20 side of Mutants and Mastermind is just because it's easy. One dice to roll and that's it. It works perfectly because the game is fast. There's no grid, no opportunity attacks (how can you handle a situation like this when you could have someone run next to you at the speed of light anyway?) and no flanking rules (that last one I'm not sure).
Powers are broken down into nice machines. You put a number in and it outputs results. I've never played a game of Mutants and Masterminds where players didn't know how to build a specific idea from the available powers.
My favorite aspect of Mutants and Masterminds is the GM fiat rule. Let's say you're playing a bunch of superheroes in New York city trying to fight an alien invasion. One of your players built this character who can do miracles with a bow and arrows. There's no ammo in Mutants and Masterminds so basically he can never run out of knives..unless the GM has something to say about it. So what you do is you gently give the player a Hero point and say: Sorry buddy, but you ran out of arrows. You can use that to call that a certain power doesn't work anymore or something happens. You give a hero point and they can use it in so many ways. The reason why this is the best part is that it's just what d20 needed specially in a game where characters can basically be immortal and invincible (with the right combo).
Where Mutants and Mastermind is running short in my opinion is the less flashy aspect of superheroes. If you want to make a game closer to Batman or The Punisher, your players must have really good self-control over the options available. A typical Grim game will be Power Level 6+ (see Iron Age supplement) but you still have access to all the powers (unless the GM makes his own list).
I've played from everywhere between PL6 (Grim City game with vigilantes) to PL15 (Players are minor gods working to investigate a major god gone rogue) and it not only works, but it's awesome. The rules are fast, efficient and when you know how the powers works you can improvise an entire BBEG on the fly.
The thing you need to watch for Mutants and Mastermind is to not let power resolve everything. Make sure they have skills because yeah you can be invisible but someone with the right gear or powers can find you so don't forget your stealth skill..it could save your life.