Ultima Online is one of my all time favorites for instanct pvp action. It's fast paced with a steeper learning curve. Typically skill is the determinate factor in this game vs "time". Look for a free server. Easy to find and doesnt cost a dime.
I can't recommend EQ2, SB, DAOC2, SWG, Lineage, Matrix Online.....
However i did play eq2 for 3 months that's 2 months longer than the other games in the "not recommended list".
My roomate plays WoW and it looks really pretty not sure on gameplay and such but he's a "soloist" so i dont think he really takes part in the mm part of mmorpg =D
and of course!! MuDs are alwasy fun