I have been mudding since about 1995. I have tried out a number of muds of varying types. My all time favorite mud is Tsunami! Tsunami currently has 58 subclasses available and about 24 races available; allowing for a wide variety of combinations for your playing pleasure.
Tsunami is very stable, since 1995 I don't think it has ever been down for more than a day at a time, and I only recall one down time due to an actual problem on tsunami and not with ISPs/routers connected to tsunami.
The code base is expansive offering hundreds of unique areas in which to adventure. While you explore tsunami you'll find just about anything you might want, from PK to RP to Hack and slash playing... You'll also find that the wizard staff is always looking to improve the game and environment!
One sample is the recent addition of the "Jihad!" Six ancient gods have entered tsunami and are rallying people to their belief in order to be the ruling faction of tsunami.
I've got to say that overall it's the funnest mud I've ever played.
Not your, Enemy.