These are always fun discussions and involve a lot of subjective judgement when comparing teams from different eras, but I think a case can be made that the San Francisco 49ers of the 1980s were the greatest team of all time -- and the 1989 team in particular stood out above them all.
The 9ers were good, and Joe Montana was one of the best. But Bart Starr and Lombardi were not only great, they defined greatness in the NFL.
It's hard to argue. The only competition is the Steel Curtain.
As a Pats fan, I'm biased, but... The Pats have to be the best team of the salary-cap era (post 1994). What Belichick and Pioli have done with a rotating cast of players is astounding.