I can't say which is the best MUD out there...but there are some quality games to be found...sadly...most of the better MUDs are also the ones with almost no-one playing them
ACHAEA
Website:
http://achaea.com/main.html
Connect:
telnet://achaea.com:23
Achaea was suggested to me by DarthWufei and was the first MUD that I ever played. The game has a GRWAT sense of direction...with detailed descriptions for all of the rooms and objects. The game is fairly large and features a lot of PvP possibilities in the form of hundreds of abilities and spells. Very few of them work against A.I. baddies though....the game also has the completely assinine credits system...which pretty much mean that you get ass-raped out of your real-life heard earned money to buy levelup tokens for your character OR you sit it out and level up the old fashioned way (for free) and play for years doing what idiots with money to burn can do in days. The game doesn't have any Auto-combat options...you pretty much ae in full control of every attack you perform and must initiate every swing, kick, spells, and ability in combat. Has a pretty ace JAVA client (as far as Java clients are concerned) that can be used by clicking the PLAY NOW button on the site). Roleplay heavy MUD. Achaea is made by a company called Iron Realms...their website has a handful of other games that they make...all seem to be similar in some areas, though Achaea is the most popular
AARDWOLF
Website:
http://aardmud.org
Connect:
telnet://aardmud.org:4000
Aardwolf is pretty popular...400+ players a night is common and there are a metric ton of skills and spells in the game. The game has a bunch of fairly straight forward gameplay systems that get the job done in a rather bland way. A pretty basic game, despite having a ton of content, if that makes any sense. Auto-combat abounds...any offensive skill intiates and you begin combat. Using skills/spells makes up the meat of a battle. Tons of chat options and player interaction can be had but most of the players don't like talking that much. Easy to get lost as many of the rooms are barely described. The lack of solid descriptions for everything was the main hurdle from me after I had played Achaea for so long, but I switched. Partly for the increased playerbase, partly for the lack of enforced roleplaying, and mainly for the lack of any bullshit credit system. All players level up the same way...there are no obvious shortcuts such as buying fucking tokens to level quicker. Donating money to the MUD causes double experience to all players connected...so even if you want to support the MUD you aren't the only player to benefit.
DISCWORLD (Down right now)
Website:
http://discworld.atuin.net/
Connect:
telnet://discworld.atuin.net:23
Just recently started playing this one...features an unsual gameworld based off of a series of books. A very descriptive game

D) featuring a Morrowind-like skill system...meaning that actions like wielding Battle Axes, Climbing Trees, Picking Pockets, and Casting Spells are all ruled by specific skills that improve with use. Your first attempt at picking a pocket will likely result in failure, but the more you try the better you get at it. Combat is of the auto-attack variety though I haven't gotten deep into the game to get a real feel for it. The ease of becomming a tutor is nice, and joining a guild gives some nice bonuses...a lot of players are on at nights...over a hundred...which is not quiet up to Aardwolf levels...but still good.
RETROMUD
Website:
http://www.retromud.org/
Connect:
telnet://retromud.org:3000
RetroMUD is a hardcore MUD in the sense that starting off in this game is really fucking hard. The game features a metricfuckton of character options in the form of player
Races and
guilds. You start the game off as a weak simpleton...you can't kill a fucking thing...the idea is to wander around areas and gain experience for each new room you encounter...a hard task due to the main drawback of a MUD: the lack of visuals. It's hard as hell keeping track of your location...a MUD Client with a strong auto-mapper is pretty much required in my eyes. I always get bored of wandering around in the two or three planets that I start off on and try my hand at fighting something...nearly always with pathetic results as my character gets raped repeated by aggressive enemies that band together and (this surpised me the first time it happened) actively hunt you down should you attempt to flee from combat. Keeping track of your skills and spells is pretty hard, as the names aren't always easy to remember and the only way I can figure out to display all of your available skills and spells is when you are in your guildhouse. The game has a lot of potential, should you manage to get over the hurdle of actually being able to navigate in a MUD...a hurdle I have problems with all the time.
Some other MUDS I frequent:
1) SlothMUD III
2) Realms of Despair
(been playing this recently...pretty similar to Aardwolf in look and feel, though I'm having a very hard time keeping my bearings...looking at my logs I pretty much spent the last 2 playtimes trying to get back to a specific area with no luck...sometimes I wonder why I even bother playing MUDs when I can't get over the issue of navigating around in them =\)
3) There are more...but I'm getting pretty fucking tired right now...had a rough day at work and all that =\
http://www.topmudsites.com/
http://mudconnect.com/
These two sites can show you a great deal of MUDs and some info to help you choose. A warning about the ratings is that most of them are rigged by idiots with vote scripts and such...so
don't take a MUD's listing to heart. Just because a certain game is 10 places above another does not mean that it is better
MUSHClient is a nice free MUD client and what I use...HTML logging, MUD specific settings, and a pretty advanced Macro system keep me happy. You can use it for free or pay for it...the free version I believe is only limited in having 6 lines of begging you to register whenever you connect to a mud and a 10 or so second long splash screenwhenever you start the client up.
zMUD seems to be another good one....bullshit or not I dunno...you gotta pay for it but there is a demo out there. Has a auto-mapper...in fact...the auto-mapper is the main reason people use it.
http://www.ga-forum.com/showthread.php?t=27493
My old MUD thread...afterwards I pretty much stopped playing online games (due to school and outside causes) until just recently