It's really a question of what you're into. MUDs run the gamut from purely social with little combat (a bit like IRC with more functionality) to traditional medieval fantasy hack-and-slash to sci-fi space combat.
Some have specialized clients they recommend; I'm a big fan of just using TinyFugue [0] in a shell window, but that's just me. :)
MUD Connector [1] is a great place to start, as it has a few thousand listed with average sizes, themes, reviews, etc. All searchable, so you can find what appeals to you.
As a very general rule, start with the bigger more active ones. The smaller ones can be very fun and you can make great, close friendships with other players, but they also can tend to get kind of insular and hard for a new player to penetrate.
Some have specialized clients they recommend; I'm a big fan of just using TinyFugue [0] in a shell window, but that's just me. :)
MUD Connector [1] is a great place to start, as it has a few thousand listed with average sizes, themes, reviews, etc. All searchable, so you can find what appeals to you.
As a very general rule, start with the bigger more active ones. The smaller ones can be very fun and you can make great, close friendships with other players, but they also can tend to get kind of insular and hard for a new player to penetrate.
[0] http://tinyfugue.sourceforge.net/ (it's also in most package managers and in homebrew for Mac)
[1] http://www.mudconnect.com/index.html