Well, I guess World of Warcraft still has the biggest following, but have not tried it myself as I'm burned out on fantasy settings.
Age of Conan looks good from what I've see, when it comes out. Might get me back into a fantasy MMOG.
Pirates of the Burning Seas looks good when released.
Some will be into Lord of the Rings Online.
You can look at what folks are playing at
http://www.mmogchart.com/.
Or browse through some at
http://www.gamesites200.com/mpog/http://www.mmorpg.com/ might help you too.
Not really into any other MMOGs right now. I've gone through Guild Wars, Shadowbane, City of Heroes, City of Villains, Anarchy Online, Dark Age of Camelot, Auto Assault, D&D Online, EVE Online, SWG.
I was looking forward to the Online version of Privateer before it got axed. Star Trek Online is gonna be a bust like SWG was. Frontier 1859 has a great concept, but I doubt it will get the funding it needs to be completed and done right. Same with GURPS Transhuman Space Online.
Too many are falling into a mindless leveling treadmill. I don't think I've ever "finished" a MMOG yet or gotten to the highest ranks..... I get bored with them long before that. When it becomes a choir, it gets pulled off the hard drive. Nothing really new and exciting coming out as everyone funds the business model that has worked so far for WoW and clones.