Montana is a bit over rated because of wins. He's good, but he's over rated by most people due to the Super Bowls - just like any QB who wins several Super Bowls.
Young, Manning, Marino - all of them aren't over rated nearly as much precisely because people aren't drawn in by their Super Bowl wins. They've got two rings among the three of them, and yet they are all easily top 10 QBs all time. I would argue Manning is the GOAT, and Young and Marino aren't that far behind.
The stats speak for themselves. Brady takes fewer sacks than most QBs. Some of that is him throwing quickly, but a lot of it is the O-line. The wideouts greatly increased his abilities with their talents. A committed Randy Moss in his prime was unstoppable - you can't honestly think Brady gets to 50 TDs without him. Wes Welker is an incredible slot talent. There's no way you can truly believe Brady would be better without him, either.