Tintin++ has been my favorite for a few years now. And there is no close second to this mud client. I can run python scripts directly from it, ssh, query an sql database, or just text my phone when I pk pops up and I'm idling.

Mudmaster2k6 was very good for it's time but wasn't supported for long.

ZMud Cmud and MUSHclient all fall into the same category IMHO.
Windows programs covered in GUI that are very sluggish. I liked that you could use C#.NET with CMud but's thats all it had going for it imo. Exchanging speed and programmablilty for --GRAPHICS--- while playing a --MUD-- was never an option for me.