I pretty much like every MMO I've played TBH (which is most of them!). However I'm perfectly happy playing MMOs for just a month or two then moving on, and don't get the whole "devoting your entire life" to them that most people do. What's the prob with buying one and playing for just that month like you would any single player game, etc? Then popping back now and then as the urge takes you.
Highlights for me over the years have been;
LOTRO - I got the lifetime sub for this at launch actually, as I had so much fun in the beta, and it's almost comical to think how much value I got from that one-off £99. A truly huge game that really makes you feel like you've been on one massive epic quest over the last 7 odd years. Travelling from the Shire to the dark depths of Moria, then coming out the other end and seeing Mirkwood, Rohan and now Gondor...just incredible. Also the massive battle system, mounted combat, etc have added some neat and original gameplay twists that set it apart from its peers.
SWTOR - definitely the best for pure solo/story content, which is my chosen style of playing MMOs. I really liked those early days of flying around the galaxy and seeing each new planet for the first time, and the surprisingly well written characters living on them. Some classes are better than others sure, but the Sith Warrior, Imperial Agent and Republic Trooper campaigns have all been incredibly engrossing, and match a lot of Bioware's single player output for me.
Neocron - anyone remember this? Not the greatest MMO by any means, but it was the first I ever got into, and its cyberpunk/blade runner setting is still surprisingly unique to this day. Real-time shooting, neon strip clubs, a Fallout-style wasteland and an awesome Shadowrun style atmosphere were enough to get me hooked on the genre forever. I believe a Neocron 3 is long in development, I hope that eventually appears.
And two disapointments;
Guild Wars 2 - I love the look and feel of this world, but I find it incredibly lacklustre to play. I think the removal of quests in favour of sterile area "tasks" results in massive boredom, and feels hugely repetitive. The class stories are great, but those missions are so few and far between. Big let-down on the whole, this - I'm still pretty baffled how many people still talk about it as the second coming when compared to some of the above.
EVE - such an amazing setting and atmosphere to this world, and the concept sounds utterly jaw dropping and basically my perfect game. But if ever there was a chasm between how something sounds on paper and what it's like to actually play, it's Eve. It feels like a genuine full-time job to play, minus a salary. This game's world, depth and complexity, married with a proper ground exploration/questing system would be stunning though.