I would have to say Brady is the best all time right now. He is headed for his 6th Super Bowl which no other QB can say. But the larger point is that he has produced and won time and time again without the very best backs in the league. He has made the people around him better moreso than any other QB in my opinion. The Patriots have interchanged a lot of parts over the years and Brady continues to thrive. Maybe the system does deserve some credit but I feel like the bulk of it is Brady. We really won't know whether it was the system or more of him until he retires.
Nothing against Joe Montana, I think he is a close second, but he did have Rice, Taylor, Clark, Craig and others. Brady didn't have that kind of talent around him. If you look at the top 10 receivers all time for the 49ers, five of them played with Montana. Did Montana help, yes. But the players I named would have been great anywhere. I can't say that's true with some Brady have worked with.