I never thought I would say a sentence like this, but:
I have to agree with Stink Fish Pot! about the Steelers, of course. 
How about the '92 US Olympic basketball team (the Dream Team)?
Granted, they only played together a couple of weeks, but still
they had arguably 3 of the top 5 players of all time (Jordan, Bird, Johnson) and 10 of their 12 players were in the NBA’s top 50 players of all time.
As for teams that were together for at least a season, the Celtics 8-peat puts them on top, I think.