My favourite's always been Gunz: The Duel, if your good...It's <3. MapleStory is unbelivably childish, and Gunbound is mega noob un-friendly. PS : Gunz isn't a 1st Person Shooter....
I would have to say.... If you got a little bit of money this is not a p2p game it would have to be Guild Wars... But if you want it totaly free I would go with RF online or Mu Online.
if you want an addicting game but terrible customer service, you can try Conquer Online. although its technically is free to play, you basically need to pay for items to get any good stuff.