From the standpoint of career accomplishments matter, Brady is clearly the GOAT. Montana still holds an edge over Mahomes.
In terms of who is the all around most talented QB to ever play the game, we've never seen anything like Mahomes. His physical & mental abilities combined with his desire to be great & find a way to win are simply a total package unlike any other. Rogers, Elway are the only QB's I could even put as a rival as far as total package.
Brady, Montana and many others can compare in one or two ways but not all the attributes.
There is simply no QB in the history of the game I would take over him in a "you can have this guy in his prime" scenario.