I have played many different MUD's over the years, but I have to say, Discworld is easily the best. A huge team of many different creators have built a massive world: for and idea of the size, just one of the many cities (Ankh-Morpork) has over 4000 rooms, much larger than many entire MUD's!
And all this size dosent sacrifice the quality of the play in the world; many years of coding on the part of hundreds of individuals has made a rich and diverse world including more than 5 different countries each with uniqe languages and cultures to be mastered, 6 different guilds (wizards, witches, warriors, thieves, assassins and priests) hundreds of spells and weapons to role play to your hearts content.
You can do literally ANYTHING, and I mean ANYTHING! Run a shop, write for the newspaper, become a citizen of a county and rent a house, publish books, learn to play instruments, forge weapons, paint pictures, take photgraphs, make furniture... ANYTHING! Even go around killing other players if you like that kind of thing.
This world, though fully playable in all parts, new aspects are always being added - new areas, objects, spells - even you can learn to create and playtest to make this rich and multifaceted world grow!
The build of the MUD is based around the Discworld series of books by Terry Prattchet, and the game reflects the feeling of huge extent and hilarity. Even if you've never heard of the books and stories, have a look and enjoy thr trip - it's huge!