I would probably say WoW.
It has the largest world, the most content, a good combat system, crafting is actualy worth it for the most part (though you can ignroe it if you want).
My main gripe on WoW is Travel time and that partying (low levels) is non existant.
My PERSONAL Favorite MMO is City of Heroes, along with the CoV. Combat is similar to WoW, though you can customize your combat style much more. If you buy both CoH and CoV it is only 1 monthly fee of $15 (nice deal there, the games realy are rather different as the Archetypes (read classes) are very different).
You can solo or party (and neither is particularly hard). If you are a solo junky it is actualy possible to solo almost ALL of the content in the game (this include Raids, though you have to be rather badass to do that). The party system makes it very quick and easy to get agroup going, you all get mission rewards, wich is awsome, etc etc etc
yah, I love CoX
(as a note, CoX refers to City of X (c/v), now all of you pull your minds outta the gutters

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