I would say Vindictus....but Nexon's service has gone down the drain
They still haven't figured out how to stop all the people getting their accounts hacked =_=...
That's about the only online game that I know off the top of my head that has realistic graphics and awesome boss fights (especially the huge ones later on~) You can always try it and see though.
Ninja Edit: Dunno if your into the final fantasy series, but maybe Final Fantasy XIV online?