Bryce Harper, 2018
While the Cardinals did not make a play for Bryce Harper while he was in free agency, this to me remains one of the biggest mistakes they have ever made.
Harper had just finished up his age-25 season when he hit free agency and had a career .900 OPS in his age-19-25 seasons while making six All-Star teams and having one of the best seasons in MLB history in 2015 when he won the MVP.
Harper sat in free agency most of the winter, eventually receiving a 13-year, $330 million deal from the Philadelphia Phillies. While this deal seems massive, you need to remember three things. First, Harper was going to turn 26 during the first year of the deal, so he was only signed through his age-38 season. Second, the deal was so long that he's only making $26 million a year, which makes him only the 19th highest-paid player in baseball this year. Third, Harper was so good, that he deserved every dollar he wanted.
What has Harper done since he signed that deal? He's arguably been better. He's had a .939 OPS since 2019, won another MVP, and was clutch for the Phillies on their way to the World Series in 2022. The Cardinals should not have sat on the sideline and let Harper end up becoming underpaid by another team.
And since he is not making an insane AAV like many of the biggest deals in today's game, the club could have afforded him, Paul Goldschmidt, and Nolan Arenado.