Apple's Mail Application
The great things about Apple's mail application is that you already have it if you're using OS X and that it's very Mac-like (for obvious reasons). Watch out if you're using it for IMAP though - for some reason Apple's Mail app opens up 4 persistent connections for each IMAP account, causing it to be a resource hog which could create a negative effect on access to your site. More information on this problem can be found here: http://discussions.apple.com/thread.jspa?threadID=1216355 - Note that of all of the apps listed, (and all known mail apps) Apple's Mail application is the only one to suffer from this issue with IMAP.
Gyaz Mail is a very customizable alternative to Apple's Mail application at a low price.
Part of Microsoft's Office for Mac suite, Entourage is an example of something Microsoft has done right. Entourage is reliable, quick and includes advanced features.
The Open-Source offering of the list, Thunderbird was unsupported by MacHighway until version 3, due to it's confounding SMTP setup. All of that's resolved now and Thunderbird (version 3.0 and up) is now supported and recommended by MacHighway. And because it's Free, you have nothing to lose by giving it a try.