My MUD of choice was DiscworldMUD (http://discworld.starturtle.net/lpc/), based on the works of Terry Pratchett and which has been actively developed for nigh on 30 years. The setting is wonderfully flavorful and the world is huge and lovingly detailed. It's such great fun to get lost in one of the giant towns and see what you stumble across.
Check out this map of what its biggest city, Ankh-Morpork, looks like: http://dw.daftjunk.com/Ankh-Morpork.png (external maps are a great aid for new players, the in-game map only shows your immediate local surroundings, so keep this site bookmarked: http://dw.daftjunk.com/ ).
I used to wake up at 4-5am to play DiscworldMUD before school.
I had at least one character in every guild and each time waking up fresh in the mended drum was like being born again.
Doing "The Run" as an assassin was just the right amount of stress and excitement and when your Wyrm sword finally stops being limp as a warrior.
Having player-character witch friends you could call to fly in on their brooms and drop off healing potions for you was old-school-cool.
I've been a gamer all my life, but nothing will, or has ever compared to the experience of playing the Discworld MUD. It's been 15+ years since I played so I don't know if it's still there, or what it's like now, but I'll never go back.
I was really, really into DiscworldMUD for a while. I played as a wizard, and had so much fun casting brassica oleracea ambulata on NPCs, then watching the cabbage follow them around. Then I'd equip Collatrap's Instant Pickling Stick and zap it into coleslaw, which I would feed to my loyal chicken. I'm not sure whether that accomplished anything, but I thought it was hilarious.
I obviously found the game very enjoyable and memorable. If I had more free time, I'd love to pick it up again someday.
Check out this map of what its biggest city, Ankh-Morpork, looks like: http://dw.daftjunk.com/Ankh-Morpork.png (external maps are a great aid for new players, the in-game map only shows your immediate local surroundings, so keep this site bookmarked: http://dw.daftjunk.com/ ).