If you could only sign one gigantic contract.....

Because as great as Brees & Peyton are, you still were a 7-9 team for several years in a row before MT then Kamara joined the Saints.
They had an impact, but the bigger distinction was in the improvement of the defense and that overall amazing 2017 draft class. When you make that statement remember that CGM came in the year before (and made Cook expendable). The year we improved we drafted - all starters - Lattimore, Ramczyk, Williams, Kamara, and Anzalone. With BU DE Hendrickson.

We also signed a pot full of defensive FAs that contributed. It was a complete defensive make over.

While there is no denying the impact of AK41 and Ram, the impact was likely felt more on the defensive side of the ball. The offense has never been less than playoff caliber ( IIRC top 5 in every year but the one they were 6th) & on-the-field improvement of the defense was huge.