If you mean free to play as in not pay a thing then I'd says perhaps Star Wars Old Republic. It went free to play but it is still very good for the character stories. I love being Sith and choosing whether or not to execute characters when they betray me (I usually do...)
If you are willing to spend a one time sum then Guild Wars me thinks? I do not really play MMO's anymore. But I do recommend those two.

If on the other hand you just want a fight game I'd say DOTA 2 on steam. Awesome game is Awesome! :rock