http://www.mudmagic.com/codes/
awesome site for MUD codebases...
I would recommend the GhostMUD codebase. It's a pretty heavily modified derivative of the ROM codebase (which is a derivative of MERC, which is a derivative of Diku). It's got a really easy to learn OLC interface that lets you edit objects/creatures/areas from within the actual mud while it's running. It doesn't really have the most freedom when it comes to coding, but it's capable of making a fun MUD and it's easy to learn how to use.
If you want to have a little more freedom with what you can code, you should go for an LP codebase. My favorite codebase is Nightmare 3, but you can't get ahold of it anymore because the original coder (Descartes) from what I've heard, is a big douche and won't allow any more licenses to the public, or something like that. But, you can settle for other LP codebases, like Lima, or Dead Souls which I think is a derivative of the Nightmare 4 codebase (which wasn't as good as 3 but was more "organized").
One of the best muds out there that still uses the nightmare codebase (although so heavily modified that they changed the name) is Core 2656 (
http://www.coremud.org). I was a coder on an earlier version of that mud (which has since been shut down), and it was really a lot of fun.