WoW-Many people hate on it, but it's the most balanced and fluid game.... sadly.
Eden Eternal- Anime inspired graphics/VERY GOOD graphics. Free to play, very balanced. Unique Class system. Open world, good dungeons, fun boss fights.
Vindictus- Dungeon based, best free to play graphics to date. Real time fighting. It's like Mortal Combat meets WoW.
Tera- wait for this! One of the most hyped free to play games ever! coming soon!
I've played every free mmo to date.... they all have pros and cons.