Guild Wars 2.
I've liked WoW before, I thought it was pretty engaging at one point. I've always had some issues with it but at a time I thought repetetive grinding, arbitrary questing mechanics and the whole experience (especially the dungeons and the raids) feeling like a historical reenactment where the player has as little input and presence as possible (rather than a huge epic story it purports to being) were things I was just going to have to live with if I wanted to play MMOs.
I might be getting Mists of Pandaria at some point anyway, just to check it out, but I'm at best lukewarm about WoW nowadays. It's a rotting house that Blizzard tries to fix by painting it over until there's more paint than wood. As the best example of this I still hold 'resilience', the PvP stat that they had to implement because of the disparity between player's health pool and the amount of damage they're doing. It's a patchwork solution, they should have done a complete overhaul of the system when they noticed it rather than implementing a stat that makes PvPing feel incredibly exclusive. Especially at early levels if you don't have the snazzy heirlooms.
And daily quests were a horrible, horrible idea to begin with, the logic behind forcing players to do even MORE of them is incomprehensible.
Other MMOs have given a fair try to fixing those issues but it's Guild Wars 2 that finally did so many things right that it mystifies me why anyone would still prefer WoW's system. Especially now since everything I've read of MoP seems to suggest Blizzard is merely wallowing in the above problems instead of trying to fix them.
Guild Wars 2 has its own issues, of course. I'm somewhat concerned about the end game, for one. Making all high-level gear of equal value and thus allowing the players to choose their own path to acquiring it is a sound move, but I'm not sure what you're supposed to be doing once you complete your gear. Besides PvPing, which is by itself pretty fun, but I do hate the fact that the World versus World incurs repair costs. If your realm isn't doing very good and you're still trying to build siege weapons and stuff, it's very easy to end up losing money in it.
The personal story is rather trite, as some have mentioned. It never got painful for me to play through but its biggest problem is that it's very short and spread too thin, trying to last through levels 1-80. When the suggested level for continuing your story suddenly jumps five levels ahead of you and you have to go do something else for a long time to catch up, it loses urgency somewhat. To raise the stakes, it makes a big deal of some characters dying along the way but they don't get enough screen time to be well-characterized, so the effect is lost. Making the primary antagonists feel like destructive, distant, inexorable forces of nature rather than moustache-twirling villains is an interesting choice, but I didn't even know what Zhaitan looked like until the very last quest.
Also the Destiny's Edge plot line commits the cardinal sin of relying too much on the extraneous novels, I feel.
Still, an MMO where even the levelling process doesn't bore me after the first character must be doing something right.
PS: I love jumping puzzles!