No. 1: LHP Josh Hader
You likely do not need me to tell you what Hader turned into once he left the Astros organization. Upon joining the Milwaukee Brewers, he immediately blossomed into one of the most dominant relief pitchers of this generation. He made four All-Star Games and even finished seventh in the NL Cy Young Award race back in 2018.
Hader has become a super-closer who possesses the ability to go more than one inning and has emerged as one of the biggest strikeout threats in the game of baseball today. Back in his Astros days, he came up as a starting pitcher. It wasn't until he moved to the bullpen that his career took off.
Of course, he was dealt to the San Diego Padres at last year's trade deadline, which was when the wheels fell off for him, but it was his dominance prior to 2022 that make his absence from the Astros that much harder to deal with.