First off, this is all subjective and I would expect some Cowboys fans to be upset that that 92 team, a great team, isn't in that 1st slot. There are so many factors to consider in each era and among them was the competition level in those specific years.
When I think of the greatest team, I consider all 3 aspects so I would not put the 85 Bears at #2, best D ever? Yes, but not the 2nd best team. The balance from O to D and the balance from pass to run has to be considered.
It's hard to argue any of the top 6 to be in that 1st spot but when I think of the competition level and strengths vs weaknesses, I would have to go with that 78 Steelers team at 1, 72 Miami 2, 84 Niners 2, 92 Cowboys 4 and 62 Packers at 5.
The requirements seem to be SB champ but I would put that 77 Cowboys team at 7 behind the 76 Raiders at 6.
I will say one thing, there's no question about the best era of the NFL being the 70's. That was the best decade of pro ball I've ever watched with a lot of good teams. 80's. 90's and 00's, not even close.