I've never played an MMO I liked.
I like the idea of MMO's. You've got a massive, ever-evolving world where thousands of players work together to accomplish epic goals. Wow! That sounds great!
I've tried several, including ones that I keep hearing are the "best" of the genre. Every one I've tried turned out to be nothing but level-grinding, convoluted number-crunching, and linear progression through ugly enviroments. They are, in short, boring.
I like the Elder Scrolls series, but unless Bethesda bucked pretty much every gameplay mechanic that has come to define MMO's over the years, I can't see myself enjoying such a game.
ADDENDUM: Drop-in/drop-out co-op would be pretty cool, though.