More MU*s, especially the non-MUD ones. If you have a time machine, the MUSHes that were around in the 90s were the best, and I highly recommend them. (If only I could keep them in bottles in my cellar.) MUCKs are also passable, depending on your interests. Heh. But yeah MUSHes, absolutely.
I love the way you've gone on about the differing quality between MUDs, MUSHes and MUCKs...
Since, actually, there isn't a dictionary-worthy definition between them...
That's completely incorrect, actually, since about 1990 or so. MUSH refers to TinyMUSH codebases (and MUX before it was merged back in), which have no hardcoded combat systems and only a simple functional programming language. Some of them have some pretty in-depth softcoded stuff to support whatever combat systems they choose to use, but a lot of them don't. In practice, and subjectively, many MUSHes are fairly small places with a strict character approval process, and strict cooperative storytelling (everyone is involved, not competitive, but must conform to the setting). Conflict resolution is usually cooperative or GM-mediated. On most MUSHes, if one character seems overly powerful and is dominating the game, you talk to an admin. Pretty much every MUSH gives some minor programming and (often) very minor building abilities to the players. Almost no MUSH runs on a modified hard-coded codebase.
MUCK refers to fbMUCK, ProtoMUCK, and other similar derivative codebases. Again they have no hardcoded combat system, but since the mid-90s they have a compiled FORTH-based programming language in addition to a softcoded functional language like MUSH. It gives them the ability to handle combat and other things that MUSH just plain can't do; they're usually more technically advanced. In practice, and subjectively, many MUSHes are larger and more open communities with less-strict character approval processes, and storytelling that is usually cooperative but often more loose or completely disjoint. Conflict resolution is almost exclusively cooperative. On most MUCKs, if one character seems overly powerful and is dominating the game, you ignore them because their world and yours probably don't intersect at all and they couldn't do anything to you even if they wanted to. MUCKs frequently give substantial building abilities to players.
MUD refers to places with hardcoded combat. Programming is usually something that the players never see. There's a spectrum of MUDs from 'strict roleplay' to 'loose, socializing roleplay' to 'no theme to roleplay in at all', but conflict resolution is almost exclusively handled by the game mechanics.
So yeah. MUSH/MUCKs are sort of like each other, but completely different from MUDs. MUCK : MUD :: Second Life : World of Warcraft.