I always like to de-bunk the "Montana is the GOAT" by pointing to his somewhat inconsistent stats and injuries. People like to say he revitalized the Chiefs in 93 and 94...but the Chiefs were already pretty good when he got there (3 straight seasons of making the playoffs and barely missing in 1989) and his arrival didn't turn them into monsters. While he was more mobile than expected, I don't remember him being great at throwing on the run. He never seemed intense to me, nor have I heard stories about him being a good "locker room guy". Montana was just kinda' "ho-hum" in terms of his locker room presence.
He was absolutely an excellent pocket passer, though. In his 89-90 seasons he looked like a beast who could carve apart any defense. Having Jerry Rice helped a lot, but so did having Roger Craig (who was also a pretty good receiver and I think the first guy to go 1,000/1,000).
There's obviously no right answer to this, but when I really break it down amongst QBs, I probably consider Johnny Unitas the GOAT. In no particular order, after Unitas, I'd probably list Brett Favre, Elway, Terry Bradshaw (you read that right. He was damn good) and Otto Graham as my top 5.
Brady, Montana, Manning, Marino are in numbers 6-10 on my QB draft board, with that last spot still being open or rotating (or, more likely: "oh sh*t, I forgot about THAT guy!"). Staubach, Roethlisberger, Tittle, Starr...they'll all land in that spot, depending on what kind of mood I'm in.
And note that this is strictly QBs. Overall top 10 players would probably only include 3 or 4 of them.
Yes, we could absolutely do one of those "NFL Network Top 10 Countdown" shows.