I really see Elway and Marino as a tie...having watched both of them through their entire careers. I'd take Brady behind both of them.
Montana and Young were both a good cut above all of them. Then Manning, then Favre. Those are the Qb's I've watched that I think are the best.
That would be the top 10 I've witnessed play. Honorable mention to Ken Stabler. Raiders leaned on the running game, but Stabler to Branch and Freddie B. will always have a special place.