This is a compliacted question. I like M&M a great deal. However, and this is a big however, it flatly does not have the flexibility in power construction that either Hero or SAS Tri-Stat have. The stock example is the Cosmic Power Pool/Dynamic Power in Hero/SAS. There's nothing to simulate that kind of thing in M&M.
Yeah, you can just make powers up. But if I want a freeform game, I'll play freeform.
Now, you may not need that extra bit of flexibility in the game you want to run. If not, then use M&M, which is a great system. If you do, then I think you have to go with Hero or SAS.
Of the two, I like Hero better. It's more clearly written, despite the larger rulebook, has far better support, and handles low-powered characters better, because of the more detailed skill system. It's also more useful as a universal system for non-supers games. If you want to use SAS for non-supers, I highly reccommend the new Tri-Stat dX book, which is outstanding, and finally fulfills Tr-Stat's promise as a universal system, much like Hero 5 did for Hero.
I dislike the new Marvel game. Not because it's diceless (I love Nobilis,) but because it's bad.