Howdy,
I started Champions with 1st edition back in '81/'82, but was burnt out of it by 3rd edition -- it became a war of "who spent their points better" and devolved more into a situation of mechanics fighting mechanics, as opposed to roleplaying and flavour. We tried Villains & Vigilantes, which is amusing for a game now and then. I also ran Gurps Supers for a while, which was so-so. For a while in the 90s my fallback was Chaosium's Superworld, which I love but only handles the gritty (read: low) end of superheroics really well. I *loved* Silver Age Sentinels, and still do - a GREAT system with a perfect balance of complexity, detail, and playability.
M&M did nothing for my group - none of us are fans of d20.
THAT all said, I feel myself being sucked back into Champions with 6E. This edition is a powerful tool and seems to have more options and is a little better with abuses. With the software to aid in character creation, it is really a strong platform for the genre.
Cheers,
Ken