WoW, without question. Don't think about Guild Wars, and don't even utter the name F---l F-----y XI here. It's alright, to be fair - assuming you like a game which forces you to party once you pass level 14, and does so all the way up to the max level. There's something to be said for FFXI but not that it's casual-friendly or even casual accessable. At least with WoW you get a good 60 levels of doing stuff before endgame hits and you need to get serious to progress further.

Warcraft hasn't become the biggest MMO ever because it's from Blizzard, or because it's Warcraft, or because it does anything especially new - it's done it because it takes everything expected from an MMO and does it better than anybody else, except for PvP. Hopefully PvP will be fixed in 1.12, as intimated (And if not then, then by the time The Burning Crusade rolls around.), but until then it's nothing special. (Of course if you never knew world PvP to begin with, it won't suck half as much not to have it, but you've not played PvP WoW until you've spent four hours fighting a constant battle between Southshore and Tarren Mill. Not for honor points, not for money, not for loot - for glory. For the Horde.)

On the downside, WoW does have more l33t kiddies and whatnot, or at least more vocal ones, but it's a small price to pay and they're easily ignored for the most part.

Guild Wars is excluded because it's not an MMORPG, it's more like a third-person Diablo, only not awesome like Diablo is.

If you have six months to devote to levelling a single character in a single job class to the max level, and are content to spent at least two months cumulatively doing nothing except looking for a group, FFXI might just be for you. If you want a game where only certain content is reliant on other people, where you can play casually and still see something worth seeing in advancement terms, and can generally have a good old time, you'll be wanting WoW.

Of course, nothing is as hardcore as EVE Online, which is totally awesome but gives me headaches. =(