#10 - Scott Rolen for Troy Glaus
Starting off the top 10 with some more controversy, a very disgruntled Scott Rolen was sent to the Toronto Blue Jays for slugger Troy Glaus before the 2008 season. If this article was written after that season, the trade would have been a huge steal for St. Louis. But in the years following, it has not aged well.
Rolen was seemingly declining offensively in a big way and was frustrated with manager Tony La Russa, so swapping him for a guy who would slash .270/.372/.483 with 27 HR and 99 RBI was incredible in the moment. But in 2009, Glaus would only play in 29 games for St. Louis, and then was done with his Cardinal career.
Rolen on the other hand, played an additional five seasons, earning two more All-Star appearances, a Gold Glove, and finishing top-15 in MVP voting for the Reds in 2010. The short-term gains of Glaus were not worth the long-term loss of a future MLB Hall of Famer.