if the pats go 19--0, it is the greatest season ever played by an NFL team, simply because no team has ever done it.
as to whether the "team" that played the "season" is the greatest ever, there can and will be a reasonable debate about that.
my own view is that things change too much over the decades to say that any one team is/was the "greatest ever." i think we can say that there are several teams that belong in the discussion of the greatest team ever. if the Patriots win a fourth superbowl this year or next i would put that list as follows:
Brown-Graham Browns
Lombardi-Starr Packers
Landry-Staubach Cowboys
Noll-Bradshaw Steelers
Walsh-Montana 49ers
Belichick-Brady Patriots
I don't put the 85 Bears or the Elway Bronco's on the list because they didn't dominate for an extended period of time. The toughest to leave off is the Aikman Cowboys, but I leave them off because it's about domination at the championship level over an extended period of time. i think that team missed its chance to be on this list due to the fight between jones and johnson.
Brown went to 10 Championship Games in the old AAFC and NFL; Lombardi won five NFL titles, including two SB's; Landry went to five SB's (four with Staubach); Noll and Bradshaw won four rings; The Montana teams won four.
I leave the Shula Dolphins off the list because his five SB appearances spanned three QB's and 15 Seasons.
it's easier to say which is the greatest team of an era. on the above assumptions, I'd say:
Best team before (overlapping) the Modern Era: Brown-Graham Browns
Best team of the Modern Era before (overlapping) the SB era: Lombardi-Starr Packers
Best team of the SB Era, before the cap and free agency: Noll-Bradshaw Steelers
Best team of the SB Era, after the cap and free agency: Belichick-Brady Patriots.