Probably WoW overall, though I can't suggest it now. I played a Tauren Shammy for a bit and it was FUN. I was a beast leveling up in the Outlands. Specced enhancement and I managed to heal better than most priests on my server. Could put in some good DPS if it was called for, too (but that was rare). But now, it's simply too easy. It's not fun any more. There's no real challenge to it.
I tried a few MMOs recently, and I'm going to have to say of all the trials I've had, Rift was probably the most fun. I can't imagine it adds new elements as it goes on, but to be honest, the rifting concept is so unique and entertaining that I can see it going on for a while. More importantly, the game itself encourages small, 3-man to 5-man teams. With the variety of classes that one can have (the game hasn't established cookie-cutter builds yet, to my knowledge, so there's a fair amount of variety there), it should lead to some interesting tactics and gameplay. Actually, that variety kind of makes up for the fact the game would seem a bit light on end-game content.
But I found it to be the most fun. There were some builds I tried that I could flat out not play effectively, and I LOVE that. My most successful build was, conveniently, an Eth Shaman (I forgot the other 2 classes, I specced all my points into the Shaman set, though I utilized a summoned faerie for healing). A bit of healing, but mostly raw DPS, melee augmented with some elemental damage. But I tried a few other builds - a weird defensive rogue, a nuking mage, a duel wielding warrior - that simply didn't fit with me. And I like that challenge. It feels like a good project to work on whenever I'm bored of that shaman.
And did I mention the rifting concept? It does so much wonder for the game itself. In a year or so, it should be popularly known as a very much viable alternative to WoW, and that competition can only prove good for both MMOs.