Best MMO I've ever played is EVE Online; If you like spaceships you should try it out.
It's also probably the most difficult and complex MMO I've ever played so certainly only appeals to a niche audience.
It is a sandbox based, so most of the content is player driven. As a result you can't expect it to hold your hand and tell you what to do all the time.
All skill training is passive, so you set your character learning something and it keeps training at the same rate whether you are logged on or off.
It also has a seriously good economy driven by the brutal pvp system i.e. your ship explodes, it actually explodes and most of the shit on it probably explodes, no magical respawn nonsense. All equipment and resources in the game are harvested/manufactured by players.
I've been playing since 2007.
http://www.eveonline.com