A Tie: Spark and AirMail
Okay, I just wrote an article about Apple Email apps here. In that, I came down firmly on the side of Spark, and I stand by that.
For nearly any user, Spark is about perfect. It's easy, clean, fast and makes managing email easy. Anyone that uses email can't go wrong with it, period. It gives so much thought to filtering, sorting and finishing email that you just can't beat it.
But I'm not any user. I have complicated needs for managing multiple accounts for multiple servers and testing and manipulating everything just so. For that, I use AirMail. It is almost too detailed, too flexible, and that's why Spark wins for easy email, but if you want power, AirMail is for you.