If by dynasty your talking about the best team with the most skill and talent then I would have to say it's the 90's cowboys. They also seem to be one of the first teams that got the concept that the offensive line might be the most important aspect of a football team behind only the quarterback.
The strange thing for me is you take time back since I have been alive, or about the last three decades the best team in that time is probably the Denver Broncos. There pretty much the only team that has been constantly good since I can remember, and most people never think about them when they think about the greatest franchisees.
If I had to pick a team that was the greatest playing by the old non-sissy rules we have today it would be the old Oakland Raiders.
The best team in one year was the 85 Bears. The undefeated Miami team played one of the softest schedules in history the year they went undefeated.