I would have to say Guild Wars. I say this because I have played just about every mmo in existence, including that god forsaken cesspool of flith, wow, and at the very least, the game play to money spent ratio for Guild Wars is in a league of its own compared to every other mmo on the market today. Yes, even wow (sorry if I hurt any of your feelings).
For a completely free mmo... try Atlantica Online.
Don't try to defend wow here kids, four years of Guild wars, with all the expansions is only $200 USD, while four years of wow with both the current expansions is $870 USD. As it turns out, there's more to do in Guild Wars and far better pvp than in the latter.