My vote is for Panther Mail. It's one of my favorite features. The message threading works great, very well designed. I use my PB all day, every day, as my main work computer for database programming and running my small consultant business.
The iCal calendar is fantastic as well. We keep multiple calendars and all the business to-dos on it. The Find feature almost lets me get rid of our tech support database for clients, it works so well.
I used Eudora and PocoMail on my Windows computer before I got my PB after I wanted something more than Outlook. I particularly liked PocoMail, which a lot of people have never heard of - but the Mac OS stuff beats it as far as I'm concerned.
And to be fair to Microsoft, I have to throw them a huge thanks for their Remote Desktop software for the Mac. Lets me work on W2000 servers over VPNs without any other software whatsoever.
I'm a very happy switcher, especially since I used and loved Macs all through the middle 90s before business needs took me over to Windows.