Definitely WoW for me. It's the only MMO I still play on a regular basis (though I do fire up GW2 from time to time, if they have something new that catches my interest). Even with some of its issues at times, it's still one of the most polished MMOs I've ever played. And plus I've been having a blast going through all the new stuff with the people I know on there. X) So fun.
Why I stay with it, aside from the aforementioned people...it has end-game content that actually interests me. I don't want to just level and then let my character languish (like what's happened with GW2, sadly), there's actually stuff for me to do that I want to do, both solo and in groups, that keeps me coming back.
I do have a soft spot in my heart for LOTRO though, even if I don't play it anymore. I played it for a while some time before it went f2p, stopped because I had started getting into WoW (after the Mirkwood expansion, blech) and wasn't going to pay for two MMOs, went back for a bit after it went free. While the gameplay seemed a like like WoW Lite (with a few extra things) to me, it still was fun, and I haven't played anything yet that has music that matches the mood of things as well as LOTRO did.