He was instrumental in the success of the Giants. Some like to say that Parcells never won without him, whether that's valid, I don't know.
I also read that in Cleveland he had an owner that could be considered worse than Jerry and hamstrung what he was trying to do. The fans torched him because he got rid of Bernie Kosar but had he stayed and implemented his plan, he might have been successful, but he retired when they moved to Baltimore. The franchise was really chaotic while he was HC.
Just realized I forgot to add the Super Bowl the Patriots went to when they lost to GB and he was DC under Parcells, so Belichik has been to 7 Super Bowls in some coaching capacity.
Regardless of whether he's the best, I don't see how anyone can dismiss him as simply being lucky or a cheater. I think his style really thrives under the salary cap era because he and Ernie think outside the box, emphasize the team and situational football and gather lots of draft picks annually to increase their odds of landing players.