Your personal needs and system requirements will determine what MUD client is best for you. Though there are a lot of great MUD clients, below are the some of the best clients out there. The good news is that all the clients below are free or offer at least a free trial!
Best Free Client
MUSHclient is one of the best free clients out there. It is full of a lot of special features such as client side aliases, timers, and triggers. It has an amazingly long searchable buffer which helps you keep track of past gaming. MUSHclient is also extremely customizable. MUSHclient works on operating systems from Windows 95 to Windows Vista.
Best Pay Windows Client
If you plan to do a lot of scripting, CMUD is the client for you. CMUD is designed for fast and easy scripting with the functionality to share them with others as well. It has one of the best automatic mapping systems of any MUD client. The full version of CMUD presently costs $29.95 with a free thirty day trial. If you don't need or use many special MUD client features then I would recommend trying a free client, but if you want to compete on a MUD that allows scripting CMUD is likely the choice for you. CMUD works on Windows XP and Windows Vista. (For a good scripting client for older systems check out ZMUD from the same company.)
Best Mac OS Client
Atlantis and Savitar
I couldn't help but give a tie in this catagory. There are much older Mac OS clients out there, but unfortunately they are either not very user friendly or out of development. I've read a lot of good things on these clients from Mac Users. Both are worth a try!
Best Linux Client
KMuddy is reccommended by many Linux users as the best MUD client on these machines. This client is relatively new but is growing in popularity. It has many regular MUD client features such as timers, triggers, speedwalking, and client side aliases. Many of the clients in this catagory can be difficult to use, but KMuddy is touted as being much easier to use than most.
Best Client for Blind Gamers
Vip Mud has a lot of the features of other clients including vibrant scripting capability, but aslo a lot of special features for the blind gamer. It is designed to work with most screen reading software right out of the box. There are also voice features such as silmultaneous voices and the ability to program different voices or even screen readers for output. Vip Mud has the functionality to gag both spam and ASCII art. Vip Mud is $30 and has a 30 day free trial.