Realms of Despair
This is definitly the best mud i have been on/in. It starts off with a brilliant newbie area where you learn the basics as you walk along the path. Apart from that the interface is easy to use. When you need to perform tasks the interface does not need long commands to be able to make your char to something... I guess to me it is the most important feature of a mud.
Further, the folks a really great and helpful all over even the newbies catch on to the spirit very quickly.