#2 - Keith Hernandez to the Mets
There was a lot that played into the trading of Keith Hernandez that had little to do with his on-the-field play, but regardless, the rest of his career made St. Louis look foolish for giving up on him.
During his 10 years with the Cardinals, Hernandez slashed .299/.385/.448 with 81 HR and 595 RBI while winning an MVP in 1979 and six Gold Gloves for the club. Trading away a terrific hitter and one of the best defensive first basemen ever in his prime aged about as well as you would think.
Hernandez would finish 2nd in MVP voting in 1984 and top-8 two other times after leaving the Cardinals, while also being awarded five more Gold Gloves, three All-Star appearances, and one Silver Slugger during his Mets career.
It has to pain Cardinals fans to think about what could have been in Hernandez was able to stay in St. Louis. Unfortunately, he got to put together a heck of a second half of his career with the Mets. If the Cardinals can learn anything from this list, it's to stop trading proven talent to the Mets.