How is this possible? 3 pages worth of posts, and no one has mentioned my (current) favorite MMO?
Try FFXI. Seriously.
14-day free trial(no credit card needed): http://www.fileplanet.com/192215/190000/fileinfo/FINAL-FANTASY-XI-14-Day-Free-Trial
#1 reason to consider playing FFXI: http://www.playonline.com/pcd/topics/ff11us/detail/3599/detail.html
FFXI has changed a LOT since the time of its' inception. Bit by bit, Square-Enix has changed almost everything within the game to make it more fun while still retaining challenge.
The three golden aspects about FFXI:
1. Social
2. Story
3. Variety
With the Level Sync function, you can party with your friends no matter how low or high level they are, and actually GAIN something from it (in both having fun and gaining exp). Given that:
1. parties are a necessity for almost all jobs
2. the community is one of the nicest around
3. the auto-translate function bridges the language barriers
It's ridiculously easy to make friends.
FFXI is a game that also prides itself on its' story. There are tons of cutscenes that flesh out a definitive story (that expands with each expansion), and there are visible benefits to completing the storyline (such as being able to ride an airship for quicker transportation). It's not the best writing ever, but it's a big step up from its' nearest competitor.
Variety is the spice of life. FFXI, with its' expansions, has 20 jobs. Even without it, it still has 11. And you can switch between them at any time, as well as have some benefits of other jobs (with the main/sub system). You could be a tank, damage dealer, healer, debuffer, soloer, anything you want in the course of a single day, if you ever get bored. Besides this, there are 9 crafts that you could level (not counting "unofficial" crafts like chocobo digging, raising, etc.). FFXI is an MMO that allows you to do basically anything (provided that you meet the requirements, did the quest, etc.) you want, and it's some of the most fun I've had in ages.