1. The Green Bay Packers of the 60's
Never went undefeated, and never had the salary cap.
2. The Steeler's of the 70's
Never went undefeated , and never had free agency or the salary cap. And sure they had a good defense but the Patriots have the greatest offense the NFL has ever seen.
3. The 49ers of the 80's
One of the best QBs in the NFL ever and the best wide receiver ever. But they never won every game and their defense was not as good as the Patriots' this year. Their offense was not half as good as the patriots either. Also again no free agency or salary cap.
4. The Cowboys of the 90's
Great team all around, but never went undefeated, and did not have free agency. Very self destructive. Great offense, good defense, but New England's offense was a lot better.
With all this being said people will disagree. Remember the best offense in history, solid defense, one of the best coaches of all time , and did I mention the best QB in the history of the NFL.
Unlike the above-mentioned teams, the Patriots can lose key players and still win when the other teams could not. They are the greatest organization in football bar none.