No. 6: Mo Williams
The first iteration of the Cleveland Cavaliers was never able to find a proper second option for LeBron James, be it an aging Zydrunas Ilgauskas or an even older Shaquille O'Neal. Mo Williams was their best swing, a high-scoring point guard who played off of the defensive attention that James drew in.
Williams had a couple of strong seasons shooting the ball but was overall never truly a Top-30 player in the league, which limited the Cavaliers' ceiling significantly. The year after James left for Miami the Cavaliers were trying to stay competitive to help owner Dan Gilbert save face, but Williams was not capable of carrying the team. When the Los Angeles Clippers came calling with the possibility of a point guard swap, Cleveland was wise to take full advantage.
The Cavaliers sent Williams to the Clippers in exchange for Baron Davis and an unprotected first-round pick. That pick famously jumped up to No. 1 and became Kyrie Irving, a better point guard by year three than Williams ever was. It was a coup for the franchise and likely a major reason why LeBron felt comfortable coming home to Cleveland.
Williams had a couple of decent scoring seasons off the bench over the next few years, but his efficency spiraled down the drain and his defense was nonexistent. He returned to the Cavaliers in his final season at age 33 and played a minor role off the bench, earning the ring he could never help LeBron James win when he was in his prime.