Mutt.
I realize your question is about what client to use, but the server is also fundamentally intertwined with your choice. Hence the following tale of victory (or woe, depending on your point of view
).
Until a few months ago, I was using the nmh suite, but when I had to solve an email problem for my wife, I wound up installing an IMAP server (cyrus) on a Pentium box built out of spare parts. I decided that using the server for my own mail was a good idea as well, so I can now run mutt from a console or from an xterm/rxvt/whatever. I can even check my mail from either of our windows boxes in Netscape/Mozilla, or I can start a Windows telnet session and run it on the Unix box.
I haven't tried it, but I
think I could even look at my IMAP email using Outlook Express on a Windows machine, but why would I want to?