I'll tell you about one I hated: Thunderbird. Damn, what a pile of crap when I tried it in 2004 and again in 2005. Haven't tried it since though, so it might be amazing by now.

I used to use Agent (also a news reader) in Windows a long long time ago and then The Bat. I can't stand Outlook (used it for some 8 years at work), Outlook express was (and is) a complete joke and Eudora has always been a PITA on all platforms - that was on my ban list 15 years ago.

I can think of things I'd change and add to Mail, but right now the only thing that pisses me off day to day is the instability and slow speed introduced with Leopard.