Om’s own choice is Apple’s Mail app.
Personally I used Mail for years and loved it but I had to drop it eventually in favor of Thunderbird because its junk mail filters are useless. I kept clicking on the Junk button to try to train it to learn what was junk from what wasn’t but Thunderbird was orders of magnitude better at stopping spam so I had to move away from it.
Reading that there is a new version of Mail in Leopard I quickly checked out its new features but I was disappointed to see no mention of improved junk mail filtering.
I don’t see any compelling reason for me to upgrade then 🙁