I've always used PennMUSH. No special reason, it's just what I learned on and it was too much of a pain to deal with the subtle differences with the others when coding. Everyone who complains about PennMUSH channels? That's me when I'm on a MUX game... OMG HOW DO I TALK?!?! Many things that we think are "intuitive" are only that way because it's what we're most familiar with.
My only requirement for a new MUSH server is that it not use MUSHCode. Seriously. I never ever want to touch another line of that stuff ever again.
(This is not to slam MUSHCode. For 80's era technology it's a pretty powerful scripting language. But by modern standards? It's like trying to code a web app in assembly language. Insanity.)