I find rating email clients to be very subjective. --Maybe more so than any other Application.
With that said, my choice is Bare Bones Software's Mailsmith. It has BBEdit's great text-munging capabilities, a very clean interface and it does NOT do (nor ever will, according to BBS) HTML mail. It's filter & scripting capabilities are second-to-none. Not to mention BBS's great tech support (should you ever have a problem with the program, need help in creating a script, or wish to log a feature request for a future update to the program.)
--Mailsmith and BBEdit are old friends. I could not imagine my day-to-day computing without them.