When arguing about the greatest teams in NFL history the only reasonable comparison is according to the historical context of their era. But if you remove historical context and if you really had them play with today's rules who would be greatest team ever???. If you enforce currents rules and take into account the size and speed of players currently in the NFL that would almost by definition eliminate teams from before mid 80's. Just consider the great packers teams being 50 pounds outweighed in both lines of scrimmage or Ray Nitchske facing and trying to cover RB, TE's and WR's in spread formation absolute disaster.
My top 5 if they actually had to play:
1) 1989 49ers: No team in my opinion had a higher level of play by the time the playoffs hit, their zone blocking scheme would reduce their their size disadvantage in the line. Add a Joe Montana & Jerry Rice in their primes, and a good to very good defense, scary to any team ever.
2) 1992 Cowboys: Out of the last 20 years I would probably say this team was the most talented and deep roster we have seen and it's probably not close. The cowboys should send draft picks from here to eternity just to try to even out the Hershel Walker trade. A team that even by today's standards has a very big offensive line add a the Emmitt-Troy-Michael triumvirate in their prime and a number 1 ranked defense, just awesome talent.
3) 2004 Patriots: The best of the patriots teams that actually won a Super Bowl, it included a Tom Brady in his prime a 1500 yard Corey Dillon, a defense with they key players still in their prime: Bruschi, Harrison, McGinest and many more. Add a spread formation offense style that would drive nuts any team from the past.
4) 1996 Packers: One of the few teams to actually finish number 1 in both offense and defense, on offense a in his prime Brett Favre with weapons galore (Levens, Bennett, Rison, Chmura, Freeman). On defense a big time defense with Reggie White and Leroy Butler. Very balanced and in general a very dificult matchup.
5) 2000 Ravens: I'm going out on a limb on this one, but a team with 16 game record for points allowed has to be considered and the greatest run defense we have ever seen. It's a team that almost by definition would defeat teams that live of their running game. They wouldn't win a tournament but they would be horrible matchup for run heavy teams.
P.S: I would have the 2007 patriots as my 3rd team if they actually won the super bowl. But lets keep it to super bowl winners.
This my list, give me yours. Let the arguing start