#9 - The Randy Arozarena trade
This trade happened back in January 2020, and at the time, received a lot of mixed reviews. Some applauded the Cardinals for grabbing a high-level pitching prospect like Matthew Liberatore, while others didn't like the idea of giving up a proven bat in Jose Martinez and a promising outfielder in Randy Arozarena. To this point, the Rays look like they won the deal in a robbery.
Arozarena would set the post-season home run record in 2020 for the Rays, hitting 10 bombs on their way to the World Series that year. The following season, Arozarena would post a 129 wRC+ and win rookie of the year, followed by a 124 wRC+ season in 2022. While the Cardinals have plenty of quality outfield options now, he would have been a great piece for them in 2020 or 2021, and even if they did end up dealing him, could have netted them a lot more in return.
The Cardinals did receive catcher Edgardo Rodriquez as well, but the centerpiece for them was the lefty Liberatore. Liberatore has still yet to establish himself as a quality big-league arm, posting a 5.97 ERA in 9 games during his rookie year last season. There is still time for him to turn this around, but it's hard to imagine him being good enough to make this trade even defensible for St. Louis.
The Cardinals traded away a young impact back with great defensive ability in Arozarena, and that's something John Mozeliak and company must regret often.