As Miami's football operations boss, he took over a 1-15 team and turned it into a division champion within a year. While his previous turnarounds weren't as dramatic as that, what he did with the Jets, Patriots and Cowboys was admirable.
Parcells' career head-coaching record is 172-130-1 for a .570 winning percentage.
Belichick deserves partial credit for Parcells' success. Every time Parcells went to a Super Bowl, Belichick was his defensive coordinator or assistant head coach.
On his own, Belichick won three Super Bowls with the Patriots and nearly closed out the NFL's most dominant season in 2007 with a fourth ring. He has overseen New England's football operations since he took over the job in 2001.
Belichick's career head-coaching record is 148-92-0 for a .617 winning percentage.
Of course, Belichick's legacy will be marked by the video-taping scandal that ensnared the team three years ago. The Patriots still almost ran the table after that. And Jets fans might hold it against Belichick that he rebuffed them to become head coach of the Patriots.
My selection is Belichick based on the number of Lombardi Trophies and the fact that without his work as defensive coordinator, nobody can say for sure Parcells still would have won his pair.
True, you can argue it's easy to run a defense with a revolutionary defender like Lawrence Taylor running amok, but that's how Parcells won his Super Bowls, too.
Some also might argue Belichick has been carried by Tom Brady, while Parcells has won with a variety of quarterbacks. Belichick did win 11 games with Matt Cassel two seasons ago, and Belichick shouldn't be punished in this debate for managing to remain in one place longer than four seasons, something Parcells has done just once.
But this question is open for debate, and I fully expect some animated comments below.