Number 1: Edwin Diaz
It really has been a painful list. My initial thought when I sat down to write this list was that James Paxton would be first. But in all actuality, Edwin Diaz would net the Mariners a better package than even James Paxton. Look at the deals swung for bullpen arms at the deadline in recent years.
Justin Wilson fetched the Tigers Jeimer Candelario. The Yankees parlayed 2 months of Aroldis Chapman to Top 5 Prospect Gleyber Torres. They also trade Andrew Miller to Cleveland for Justus Sheffield and Clint Frazier. Absolutely crazy deals.
Here is the thing though: Diaz is younger, has more control, and has the same upside as Miller and Chapman. The Mariners have every reason to ask for an Andrew Miller like package for Edwin Diaz. They may not get it, but it is a reasonable asking price for the best young reliever in all of baseball with 4 years of club control remaining.