I found Entourage worked the best with my employers imap system, but I'm not fond of the interface. Since getting Tiger I've moved to Mail, like some of those above. It works very nearly as well - just one irritating thing, it keeps trying to get access to some inaccessible mailbox called "spam sample" after getting all the mail from other folders, then sending 2 messages to say it can't manage it. While the messages are up it won't accept typing, so you find the last few sentences you've typed haven't been entered. Otherwise it works excellently and the interface is very good indeed, with really useful threading arrangement and of course spotlight. address self completion is very good too.
Thunderbird was pretty good as well. I've gone off Eudora somewhat after having a nightmare transferring mail from windows eudora over to the Mac, apparently its famous for making transfers difficult.