#1 Lineage 2
Castle sieges are all about strategy. I've never seen so much teamwork in a MMO like this one. Antharas was a big freakin rush, 150+ people all strategically assigned specific jobs, If one party fails to do a task the entire 150+ people suffer. Don't play private servers, its not worth your time.
www.Lineage2.com - LineageRadio? Neat.
#2 Earth and Beyond (discontinued)
This game promoted exploration.. space exploration, even surface on planets... Awesome... First one of its kind. Its unfortunately they took down the servers.
#3 WoW
Big world, lots of players. Quests are well thought out. Plenty of RPers, makes a nice "active" environment. The graphics seem out of date... 16bit color?