There's a lot of good MUDs around, but I would reccomend the
Discworld MUD. It's based on the Discworld books written by Terry Pratchett so it's full of british humour, so it might not be for everyone, and it does not fully follow any classic RPG ruleset - it's more like Dungeon Siege: the more you do something, you are better at it.
Take a look, and tell me what you think :)