Realms of Despair
Realms is probably the only MUD of this kind I've encountered that would have more than 10 players online on average.. A hack & slash MUD, not too overloaded with "realism", concentrated instead on gameplay and having fun. There's this feel of freedom and general "closeness" between players I haven't encountered on any other MUDs I have played... While it might be difficult and somewhat boring for a beginner (game really only starts at the "avatar" level 50), there are many guilds that always welcome new players, as well as a special council dedicated to aiding those in need of assistance.
Realms is an awesome MUD if you just want to do what you want without any enforced restrictions.
Want to get some new item? Grab a friend or two and go smash the mobile with it. Want to pkill? Log your deadly and more often than not there'll be someone to compete with; or venture into one of the several free-pk arenas. Want to explore a new area? There are dozens, with thousands and thousands of rooms, many different mobiles, various puzzles and area quests.
Immortal-ran quests are also frequent, often happening several times a week, with special "glory" rewards given to participants; and mobkill, while seemingly fairly simple at first glance, is more fun than on any other MUD I ever played. With hundreds of mobs to fight, each having their own strengths and weaknesses, each requiring a different approach, there is always something to do.
Multiplay is allowed, letting those who enjoy this sort of thing to run their own army of characters; though this doesn't mean one can always get anywhere on their own - to really advance in the game one would need to establish firm links with other players, gain their trust and respect; relationships between players are often complicated, with all sorts of politics ongoing between different groups.
Possibilites are many, limitations are few; for anyone who enjoys mobkill and/or playerkill in a free, friendly environment, Realms is one of the best MUDs out there to play.
For me, it's _the_ best.