1. Johnny Unitas
2. Joe Montana
3. Tom Brady
4. Peyton Manning
5. Dan Marino
Interesting choice. Johnny U is definitely one of the best and MAYBE could have played in any era but that's like saying Jim Brown was the best running back of all time. Too much "what if". Know what I'm trying to say?
Jim Brown had a career average of 5.4 yards PER CARRY. And that's with a 12 game season, and wider field. Defenses were also tougher back then, and it was before many of the rule changes we see now, which would have made Brown even better.
If you're going to defend Jim Brown don't forget he led the league in rushing every year he played of his 9 year career if I'm not mistaken. Correct me if I'm wrong. The defense thing is up for debate. Athletes are bigger, faster and stronger today. Running back is a postion I would give my opinion on. The best back to "play the position" IMHO is Sweetness without a doubt.
Edit: Btw... I'm a diehard Cowboys' fan. Worst defeat to date the Cowboys ever had by spread was 44-0. '86 Bears.