Edwin Encarnación

From 2011-'16 Encarnación solidified himself as one of the premier power threats in the game by hitting 210 home runs to go along with an OPS+ of 141. He was a four-time All-Star and hit 34+ home runs in five of those six seasons, topping out at 42 in 2012 and 2016. He helped lead the Blue Jays back to the postseason in 2015 and 2016 and he hit a walk-off against the Baltimore Orioles in the 2016 Wild Card Game.
The deal: Toronto GM Ross Atkins signed Kendrys Morales to a three-year contract after the 2016 season which effectively blocked Encarnación from returning to Toronto. Encarnación signed a three-year, $60 million deal with Cleveland.
Did it work out: Not entirely. While the Blue Jays got good production out of Justin Smoak, the Blue Jays would have been better off keeping Encarnación over Morales. Both guys ended up playing more games at DH over the next two seasons, with Encarnación having 70 home runs and a 123 OPS+ while Morales hit 49 home runs with an OPS+ of 103.
Encarnación produced 7.1 bWAR from 2017 until he retired after the 2020 season The Blue Jays could have kept him around for five more years and he would have been more productive than the players they had.
If Toronto had held onto Encarnación and he would have produced at the same clip he did elsewhere, he would have retired as the franchise leader in home runs, while being second in RBI, second in bWAR and fourth in hits.