In my opinion Joe Montana is the greatest quarterback of all time.
From what I understand Tom Brady agrees with that assessment as well. Joe Montana never lost a Super Bowl game. He was always "cool" under pressure and never lost his temper. Opposing teams were scared even when they had a lead if there was still a minute left on the clock for Joe to work with.
Don't get me wrong I think Tom Brady is the closest thing we have to a Joe Montana in this era but he's not better than Joe. :)