Three MUDs I've actually enjoyed:
Achaea- You all know this one. It shows you this huge world with great descriptions and a really neat method of guild advancement(You actually had requirements, some of which were really crazy. In order to advance in the Mhaldorian religion you had to do something evil. What you do is up to you, the default is sacrificing a human child, but it's a bit on the disturbing side.) Of course, once you feel like you could never quit the game and it owns your life, you reach level 20, and leveling slows to a crawl unless you start buying stuff. A level 100 character with good gear can cost upwards of a thousand dollars, which is beyond insane. I still had some good times though. I still remember when I took my test to advance in the knight's guild of Shallam, and found out "Spike Boots" didn't mean boots with spiked cleats. I had been running around for several days, a male knight, wearing stiletto heels. Ohh that was embarrassing. It only got worse too, after completing the written examination, I had to travel across the map. I thought I knew how to get there, but I stopped paying attention, and walked right past the exit, and into a huge battle between Mhaldor and Shallam. I only realized what had happened when I saw a Bloodworm right in front of me. Whoops. Combat is descriptive, but the good spells are only usable in PvP, and PvP itself is garbage. There are like 50 status effects, each with several cures, and there's no way you can use them in time in an actual game, so they just let you use triggers. This leads to a situation where you see an enemy, type a letter, and watch combat messages fill up the screen and start scrolling. Intense...
Armageddon- This one was awesome, post-apocalyptic MUD in a huge desert. While it's post-apocalyptic, technology is sub-medieval, with metal being beyond rare. Most weapons and armor are obsidian or bone. The roleplayers are epic, to the point of actually being scary. I was once accosted by a partially decayed elf with glowing eyes and sharp teeth. It turned out when I was running through the slums that I had reached the East Side, and humans aren't welcome in the East Side. To exacerbate things, I had killed someone. I threw a punch, he bit my neck, and I passed out. I bet he was undead. Anyway, I woke up in a deserted house, and he told me to get down on my knees and beg. My roleplaying skills shattered under the pressure, and I ran through a spell-effect(Which I now believe to have been an emoted cantrip, not an actual spell) and got away. Scary stuff. That reminds me, magic is really rare. Only the weakest spellcasters can be played by a starting player, the rest you have to unlock by roleplaying well. Magic is illegal, and if you get caught by a templar, you can expect to be executed. You might even get to fight a Gaj(Insectoid creature with crazy jaws and tentacles that supposedly whip your psyche as much as your body, I never got to see one fight, but the city I lived in had one for their arena). Once I started getting into it I got into some more rewarding roleplaying exchanges, like the time I pretended to be a spice(illegal drug) dealer and mugged my potential clients. I failed pretty hard, and I think I got arrested for it. Or there was the time a half-giant(Who was also a city guard, and therefore above the law) stole my sword. I can't remember what happened exactly. I think I passed out in the desert and when I woke up my sword was gone. Later I saw the giant(Who I had seen earlier) at the weapons-dealer, selling what was very obviously my sword(I can't remember exactly how I knew, but it was totally mine, and when I asked him about it, it became obvious. He feigned ignorance, and I would've thrown hands with him, but as said before, he was a half-giant. He would've thrown me down, curb-stomped me, and sold the rest of my stuff. The best thing that ever happened was the time I held up the bar. I pulled up my hood, walked in, and pulled out a crossbow, and started trying to be intimidating. Apparently I wasn't intimidating, or they were just bad roleplayers, because they ignored me, and then attacked. I shot, missed, and ran away. I passed out in the desert and went AFK. When I woke up, I was on the back of a kank(Riding ant) and the person from before was taking me somewhere. I apologized for being AFk in ooc chat and she said IN OOC CHAT that she wasn't going to kill me. She threw me in a pit! I died when I hit the ground! It's a really fun game, and by far the best MUD I've ever played. The only problem is characters have to be approved beforehand, and on a slow day it can be several hours before you get approved. The first few characters will probably die quickly, too.
Accursed Lands-I'm starting to ramble, so I'll cut this one short. It's a roleplay enforced MUD with forests and stuff. It's got a nice parser, and you can build stuff. I don't remember much more.