Not only was Alex Reyes the Cardinals' top prospect for multiple seasons, but in 2017, he was the 6th best prospect in all of baseball.
MLB.com said that "When he's healthy, Reyes is arguably the best pitching prospect in the game, with his combination of elite stuff and missing bats all pointing to a future as a frontline starter". Reyes got his first taste of big league action in 2016, posting a 1.57 ERA in 12 games (5 starts) with 52 SO in 46 innings of work. It really looked like Reyes was going to be the next great frontline starter in baseball.
Reyes basically didn't play the next three seasons due to shoulder and elbow injuries. He looked pretty good during his two seasons back from injury in 2020 and 2021, becoming an All-Star closer his last season in St. Louis and locking down the backend of the Cardinals' bullpen for most of the season. He fizzled out toward the end of the season though, and then missed the entire 2022 season with an injury before being non-tendered this offseason.
Reyes was signed by the Los Angeles Dodgers this offseason and is now rehabbing with the club, hoping to join their bullpen at some point this season. If Reyes can get healthy, he's still a talented reliever, but overall, it is a huge disappointment to fall off from being a future ace to an injury-riddled reliever.