Dark and Shattered Lands
DSL is an incredibly fun and addictive MUD. They have a lot of features that I never found at any other MUD - the sailing code, the ability for ocean battles, and the Algoron Gladiator League where you can sit in the arenas and bet and watch - or try your luck as a gladiator!
But mostly, I love this MUD for the rp and I think one of the most unique rp opportunities, at least the one that attracts me most, is their priest system. As a priest on DSL, you have the possibility of advancing in priest ranks after reaching 51 (max. level). You can only do so by pleasing the god you worship. I can honestly say that some of the most incredible experiences I have had in my rp there is when I have been visited by the god my priest worships. It's a wonderful experience, and just one of many in the Dark and Shattered Lands!