I've been playing all manner of MU*'s since I discovered them back in the early 90's. Muds, Moos, Mushes, Muxes, Muses, they've all come and gone through my telnet line, but only one has captured my interest to keep on playing in todays age of pretty graphics that abound. FiranMux.
The game is incredible in it's complexity, allowing you do just about anything from fighting mythical creatures to being a crafter and making fish stew. This game is heavy Role-Play, so if role-playing isn't your gig you may want to steer clear, but if you want RP, you'll not find a better place. Nightime player loads get close to 200 frequently.
I can't think of what else to say other than give it a shot...you won't be sorry.