I played City of Heroes for a very long time. It had great character customization, very fun and fast-paced combat, a great player community, cool zones, and nifty features like sidekicking/exemplaring (allows folks of vastly different levels to team up), adjustable difficulty, and player-made missions. I've gotten 6 characters to the max level (50), 2 blasters, 2 brutes, and 2 scrappers. I strongly recommend the game if you're looking for a fun super hero MMO, just note that its endgame content lags behind games like WoW in terms of amount of stuff to do.
I'm currently playing World of Warcraft. Its a fun fantasy game with a plethora of content and places to explore, though most of it is rather easy. At max level one can raid, run heroic dungeons (level 85 versions of the newer dungeons, 2 older troll-infested dungeons and 2 of the low level dungeons), or PvP. I like WoW's PvP, despite its imbalances. There are 4 main venues for PvP, Tol Borad (large-scale team battle), battlegrounds (objective-based team PvP), arena (team deathmatch), and world PvP (mostly PvP servers... spontaneous PvP that can happen anywhere). There are also duels. I've got 4 max level characters (85), a paladin, rogue, warrior, and hunter. I've also got a mage that's closing in on max level (currently 79).