0 Best email client for mac?

I switch email clients regularly. I've tried Entourage, Apple's Mail, Opera and now I'm just using Gmail to check ALL of my email (I've used Gmail the longest). Some of my requirements include,

• Managing about 4 accounts, address books, etc.
• Integrate calendar into mail client
• Integrate tasks into mail client
• Receive mail from multiple accounts AND send from multiple accounts
• Apply different signatures from each mail account
• Create and manage mailing lists

One of the features that I don't like about Gmail is that their Address book is very basic. I also like that using Gmail, I can check my email ANYWHERE. But more and more, I find myself needing to manage mailing lists for business and personal use. At work, we use Outlook (on an Exchange server) and I like it! Entourage is (kind of) the Mac version of Outlook, but I had some problems with it in the past ...

What are your thoughts .... what Mac email client is best for multiple email accounts (IMAP) for both business and personal use??