Personally, I favor a client called gMUDix, it's fairly simple to use and works well and supports a surprising number of extras in the way of triggers, aliases, timers, etc. It's written for linux, but runs under windows too (with a small dos window that pops up and can be safely ignored since it's just voicing complaints about expected fonts and such but can't be closed without also shutting down the client), but of course, I live primarily in linux and only use windows when I'm traveling or playing windows games for the most part. *shrug*
I've tried out a few others, and the mud magic client isn't too bad, though it's primary author isn't worth supporting.. Zmud is a really popular one, but it's pricey, compared to free, and has some other issues as well.. besides, it doesn't run under linux so well and has far more bells and whistles than I have need for.
After those, well, if you don't need aliases/triggers/etc in your client, nothing beats straight telnet, particularly under linux.