It's very possible Reid is better. He is now, I think. He's always been very good but earlier he struggled with things like clock management and in-game decisions that Belichick was great at.
I don't know if 'the best coach' exists as thing outside of narratives we create. There are very god ones, and then it all comes down to situation, players, the state of the league and how their skillset fits in with all of that.
What we can say is that Darth managed all the variables he was given better than anyone else did, created a dynasty in a league built for parity and won 6 titles. So the narrative that Belichick is the best is pretty unassailable, whether it's true or not.