Easy: if I’m game-mastering, then it’s Chad Underkoffler’s Truth & Justice (Atomic Sock Monkey Press) with a side of Tim Gray’s Legends Walk! (Silver Branch Games).
When it’s time to play, I also greatly enjoy Mutants & Masterminds, any edition (Green Ronin), Silver Age Sentinels (Guardians of Order), and Marvel Superheroic Roleplay (Margaret Weis Productions). I’ve been eager to try Steve Kenson’s ICONS as well (Ad Infinitum Adventures). Finally, I always have a great big soft spot for Underground (Mayfair Games).