I think together they are practically unstoppable. Neither are considered one of the best of all time at what they do without the other.
End of the day, players play and coaches coach. A great player can lift a good team and make them better. A coach can coach up a less talented team to be competitive.
But to win championships at the rate they did both have to be great.
But Brady is more important. He took less money over 19 years and got the other players to buy into what Bill was selling. Without buy in from players a coach is toast. He got it from Brady every year. Like Danny Amendola said, The Patriot Way was Brady not Bill.
As for a loaded TB team they still only won 7 games without Brady.