I have literally never played one, so I can’t comment on any super-power game systems or settings. I have however played a couple of abortive play-by-post freeform games over at RPoL. Both games fell apart quite quickly, but one of them included one of the more interesting and different characters I’ve created for a game.
Yukio Fujii was a Japanese horror-writer. Not well known outside of Japan, but fairly famous/well-known as far as horror-writer-fame goes inside Japan, he had a bit of a cult following but hadn’t written anything in some time. He was working on a new novel and was going to set it in America both to try something different and also to try and break into the Western markets. Painfully shy, he somehow managed to work up the courage to accept an offer from a fan who ran a bed and breakfast in a small American town to stay there as long as he wanted.
The game started off with everyone in the town disappearing and him having strange dreams about thick black tentacles bursting from his stomach. Sadly it died soon after all the characters met up, before we really got to explore the mystery of what had happened to the town - and ourselves. It was ostensibly a supers game because we clearly had powers, but the game fell apart before we really got to explore that.
Which reminds me, I’d completely forgotten about Psi*Run.
Psi*Run is a game of amnesiac psychics on the run. You escaped in some fashion and now people are chasing you. You have super psychic powers that aren’t entirely under your control and missing memories you are trying to regain, all the while trying to stay ahead of those chasing you.
Psi*Run has an interesting dice mechanic in that you roll between 4-6 dice based on circumstances (if what you are trying to do is using your powers, add a die, if it’s likely to cause you harm, add another) then you put each die in a narrative box that decides the outcome. It’s sort of like a narrative point-buy system whenever you do something significant - otherwise it’s just freeform. I really enjoyed both running and playing it so I think I definitely have to crown it as my favourite supers game - and not just because it’s the only one!