I have been reading a lot about all these different protocols which seem to offer along the same features. (MSDP, GMCP(atcp2) MXP, ZMP). Which is the dominating one? I am currently building a MUD server and am wondering which one to support.
As Runter said, GMCP probably has the largest amount of players using it, but only due to the fact that some of the largest MUDs are the few who implemented it (IRE games IIRC). MSDP is gaining support from a few people, and some neat things have been accomplished with it such as KaVir's GUI interface. MXP has sporadic support, but is mostly broken (imho) as the few clients that do support it seem to have wildly different ranges of what they support and exactly how they want it formatted (spec? what spec!? hah!).
I have been reading a lot about all these different protocols which seem to offer along the same features. (MSDP, GMCP(atcp2) MXP, ZMP).
MXP is (primarily) in-band, and offers features the others don't. It's worth implementing in addition to one of the out-of-band protocols.
Which is the dominating one?
ATCP is the most widely supported, but only as a fallback option or for legacy reasons.
GMCP is the primarily protocol for Aardwolf and the five IRE muds, so it has a lot of players using it, as has already been mentioned.
MSDP is used by ten muds so far, but they're all pretty small. However there are some larger muds that are planning to add it (Realms of Despair, SlothMUD, etc), and I expect it to continue gaining in popularity.
I've released my MSDP snippet into the public domain, and tried to make it as easy as possible to install, so it can save mud owners quite a lot of work. I would suggest having a look at it even if you wish to create your own from scratch, at the very least it'll give you an idea of the effort involved.