I would say go ahead and give Vanguard a go. It's a great game and the population was decent last I looked. Not incredible. Based on the recent changes (removing all leveling/race/class restrictions for F2P characters, allowing F2P players to have houses, etc...), I would say the population is continuing to rise atm.

Other than that DCUO is probably the most populated at the moment. I have heard good and bad things about Everquest 2. People say its not near as great as EQ was, but at the same time I see a lot of people saying they play it (and enjoy it).

Of course, if you're not looking for an MMORPG, Planetside 2 is definitely a great option.