Eve. So deep, so complex, so rewarding and soul destroying at the same time. Beutiful game. Just say goodbye to your life. (and dont get put of by the steep learning curve, its worth it)
The only MMO that keeps dragging me back is WoW, tried city of heroes, EVE online, guild wars and various others. If i had to recommend one apart from WoW it would probably be eve, steep learning curve but it is a very challenging and fun game.