What mail program offers spam filtering and a unified way to check multiple mailboxes? I have been using Mail and I really don't like it. I have tried Thunderbird, Entourage and Postbox and do not like any of them either. 

I need a clear and unified way to check all my mails and also a very non-intrusive spam filter.

Edit: The thing, that I hate about Mail is:


Its hard to get a collated view of all my mails. I have multiple classifications, that does not make much sense, some mails end up in Important and Junk at the same time. What is the point of having both Junk and Spam.
Sent Items are not maintained properly. I would expect the items sent from other mail client to be visible.
The Junk mail algorithm is faulty and I have missed a whole lot of important mails recently.
Maintaining multiple account is difficult. I have found myself sending same email thread using different email accounts. ( I think mail has memory on what is the last used email account)
Mail threads are difficult to read. It creates multiple copy of the mail thread.


What I would like in the optimal email client:


Simple, single unified view.
Seamless handing of multiple email accounts.
Ability to handle Hotmail and Microsoft exchange accounts.
Better Junk mail classification algorithm.
Maintain better record of past conversations, including sent mails.
Good interoperability with Calendar and possibly other related services (like chat client).


Edit 2: I had further analyzed my problem with Mail and I have found out, that the problem has been with Gmail's interaction with Mail. I was quick to judge, Mail as a faulty piece of software. I have found an very useful link, that has addressed, almost all of my problems, http://www.macworld.com/article/1157846/mailgmail.html. Sorry again, for a very nebulous question and being very judgmental. (I am a programmer, judging things is in my genes ;) )

Also I have been  trying SpamSieve and the faulty behavior of spam filter is mostly user error. In this case, me providing faulty training data to the software, by clicking on all junk emails and not marking enough mails as junk.
Its hard to pick a best it really is up to what works best for you! I would recommend searching around for more mac e-mail clients using your favorite search engine and try as man as you can.  This Wikipedia list will get you started Comparison of email clients for some more e-mail clients to research.