I have played many different muds, as well as some graphical MMORPGs (Asheron's Call and Dark Age of Camelot most notably). No other game has held my interest the way that BATmud has for so many years now.
I first came across it in 1995 and with the exception of a few times when I've taken breaks I have been there ever since. The gameplay is always changing with a great team of wizards and archwizards doing things to keep it interesting for older players and at the same time making it more interesting for new players.
There is a staggering number of areas to be explored with many secret rooms, quests, and 'epic' monsters to fight. It has an enormous number of races and guilds to choose from with a huge level of customization available.
If you are interested in muds and mudding, give batmud a try, the new explore system makes it easier than ever for new players to get started.