2. Marcus Semien
The White Sox let Marcus Semien go way too soon back in the day.
The Chicago White Sox should have never let Marcus Semien go. He was traded to the Oakland A's in the deal that brought over Jeff Samardzija. Of course, Samardzija was horrible for the White Sox most of the time and Semien became a star with the Oakland A's.
Semien only played with the White Sox for two years and he never played more than 64 games in either of them. His numbers weren't great but he was clearly a late bloomer.
It took some time in Oakland as well but by the end of his tenure there (and his one year with the Toronto Blue Jays), he was one of the best players in the American League. It earned him a huge free-agent deal that he is now playing on with the Texas Rangers.
An honorable mention on this list can be Chris Bassitt who went with Semien to the Oakland A's for Samardzija. He became a great starter that the White Sox would have loved to have.