Mutants and Masterminds is the most versatile (imo) system for superheroes, but the character creation is a pain. Aberrant is good, but you're not likely to find anything on here, from what I've seen (I've yet to see a single Aberrant game on Roll20. Hopefully there will be an uptick when the new edition comes out). Masks is okay, but super simplistic so if you're looking for something where you really get to let your imagination be the limit, I wouldn't suggest it. If you just want something simple and quick and don't mind how limited it is, though, it's not a bad system. You can also look up Marvel Heroic.