First off, MMOs that you have to pay for are almost always better than free ones. The monthly fee goes back into developing ways to make the game better.
If the free games were just as good, nobody would play the paid ones!
Of the MMOs that you have to pay for, I've tried:
World of Warcraft
Age of Conan
Warhammer Online
EVE Online
Of these, I liked World of Warcraft the best.
EVE Online is also very intriguing, though I think I like the idea/concept more than the actual game.
