I've tried GURPS, HERO, True 20, Modern d20, Fuzion and I playtested one called Ace (from a SJG games designer) which never saw publication, as far as I know.
Given the history of HERO, I would also ultimately consider Mutants & Masterminds a generic game system.
Of them, HERO has become my favorite, M&M my second. True 20 and Modern D20 are both enjoyable.
Ace was good- why it wasn't published, I don't know. (Probably because SJG didn't want to produce another generic rpg- splitting the market- or an anti-compete clause in someone's contract.)
GURPS, I don't care for. What little I know of Savage Worlds holds no attraction for me, even though its derived from the brilliant Deadlands game. Fuzion is one I really dislike, and for a while, it was the only system in which "HERO" products were available. Dark days for me, indeed.