If you're really looking into MUDs then you should probably start here.

While the ranking aren't exactly a fool proof method to determine how good they are, it is a way to see how popular it is and the general population at any given time.

Example: TEC is on the list, but quite far from the top because they sold their soul to Skotos games and became Pay to Play for financial reasons. This limits the number of people who play it and thus puts it far from the top. On the flip side Achaea is like a Korean MMORPG in that it is free to play but you can purchase perks with real money. Since its free more people will play and it will get more votes from the players.

If you use the Info links you get a brief run down on the MUD itself including what kind of code it uses. Most of the good MUDs have custom codes that the developers tailored to the kind of game play they wanted, but a few use open source codes that they modify in order to fit their ideas.

Of course, this only covers the open MUDs. All underground MUDs are, as a rule, custom built by the designers to fit what they want to do with it. An example would be the complex crew system of the Star Trek MUD that a former clanmate directed me to where you use programs designed for MUD connections to enter an IP address and a password for your connection. Still, MUDs are the only place where you will have Aramada style space battles with capital ships crewed by 20-50 real live people each covering a seperate diagnostic and role sphere based on their duties on that ship.

MUDs may not be as pretty as the MMORPGs of today, but that lack of a graphical strength is made up with pure computational power. When you don't have to draw models and worry about movement, keeping everything looking the same, animations, and special affects you can pile more power into more detailed operations, actions, and including more people in the fun at the same time without getting laggy.