I'm not Jaffa, but I have played quite a few muds at some time or another.

I enjoyed
Ancient Anguish [anguish.org 2222] (lp mud)
(smaller sized mud, separate races, guilds, Classic D&D classes (necromancers, rogues, mages, fighters, paladins, rangers, shapeshifters)

Batmud [bat.org 23] (diku mud)
(Huge Finnish community robust player killing community, not very easy to get started, but very very open ended and very large)

Zombiemud [zombiemud.org 23] (diku mud)
(Huge community, very open ended, easier start but similar to batmud)

3 Kingdoms [3k.org] (LP Mud) I am also currently playing here
(Large community, combined guilds/classes, no races, large variety of guilds and powers. Some classic D&D- Mages, necromancers, Knights, monks, priests. Some Chaos - Jedis, changelings, breed. Some Science Fiction - Fremen [Frank Herbert Dune style], Juggernauts [Battletech] and some others.)

3 Kingdoms seems to have kept my interest the most out of those, mostly because it is an LP mud, and also for the huge variety of critters and classes. But any of the above would be fun, depending on your desired level of challenge.

Peace

Will