As far as I'm concerned, Brady is better than Montana (Montana did it with far superior weapons at WR and RB) in terms of present-day accomplishments. He's better than Aikman, IMO, who had excellent talent at the skilled positions. And I'd say he's on par with Elway, though slightly below because Elway accomplished more with less than Brady did. (Remember, Elway got three slightly average Denver teams to the Super Bowl with about the same talent at wide receiver as Brady did and with an inferior defense compared to New England's D.)
(BTW, I consider Elway the greatest QB of the modern era.)
Brady could have run the Cowboys offense of the 90s and the 49ers offense of the 80s and 90s. I don't know if his scrambling ability is on par with Elway's.
When his career is over, he will be a legend like Staubach, Montana, Elway and Aikman.