At the moment, none.
First MMOG was WoW, I played that for six months, got bored. I tried it again last year at the insistence of a mate who's very much into it, it felt like the exact same game I left, didn't play it for longer than a couple hours before I was bored again.
Second MMOG was LotRO, that was very fun. Almost too fun, I ended up burning out on that after a bit over a year.
Third was Champions Online, I got a six month sub to guarantee beta access into my next MMOG, I played it for about 3 of those months, It just didn't draw me into it like a superhero MMOG attached to a comic IP I actually know.
Fourth was Star Trek Online, I will preface this one with the simple fact that I am a hopeless Trekkie. I got beta access to that thanks to the aforementioned six month sub to CO, I loved the beta. Got a lifetime sub, bought about 5 different copies for the various store only extras (a difficult prospect for someone not in the US who doesn't have easy access to the main stores that get exclusives). easily spent well over a grand on it. Overall i found it fun (I say that with rose coloured trekkie goggles on, really it's a very average game), but it lost it's charm over the years as the devs slowly turned into Ferengi.
Fifth was DCUO. This should go down in history as how to absolutely destroy a sure thing. It proves beyond any doubt that SOE will never learn from past mistakes or even past triumphs. The game itself is amazing, I cannot stress that enough, but it suffers from being run (into the ground) by SOE.
Sixth was TOR. I love the story, IMO they could easily have released this game as KOTOR 3, 4, 5, 6, 7, 8, 9, etc. and I'd have bought every one. But after the story there isn't really all that much that kept me there.
I've tried lots of others, but nothing long enough to warrant a mention.
Next one will probably be the Elder Scrolls MMOG. Even my mates that are glued to WoW are excited for this one.