No. 4: Kawhi Leonard
Three things can be simultaneously true. First, star players almost never leave a team with which they just won a championship. Secondly, Kawhi Leonard joined the Raptors via trade in 2018 and all parties involved assumed it was a one-year rental, even if most hoped for more. Finally, if given the chance to do it all again, Toronto and its fans would do it all over again.
Leonard and the San Antonio Spurs organization were feuding in 2018 and it led to Gregg Popovich and the Spurs trading the 2014 NBA Finals MVP that summer. With Leonard's health in question and on an expiring contract, the Spurs didn't look for a godfather offer of picks and prospects, instead accepting Toronto's relatively mild trade offer: All-NBA wing DeMar DeRozan, center prospect Jakob Poeltl, and a single first-round pick expected to fall at the end of the draft.
With Leonard (and the sharpshooting Danny Green) in tow, the Raptors surged through the regular season and fought their way through three consecutive difficult foes. They took down the Philadelphia 76ers in seven games, including the most iconic shot of Leonard's career, the buzzer-beating corner 3 that bounced on the rim four times before dropping. Then the East-leading Milwaukee Bucks went down in six. Finally, Leonard led the Raptors over the injury-riddled defending champion Golden State Warriors to win the first championship in franchise history.
The Raptors had a great mixture of established veterans and young rising stars who could keep the window open. Leonard could have returned to Toronto and competed for another title; they may even have been the favorites the following season with Kevin Durant leaving the Warriors. Everyone hoped Leonard would return and defend their title.
Instead, he walked out and joined the LA Clippers, where he has won only three playoff series (one of which he was injured for) and never sniffed the success that he had in Toronto. He always wanted to return home to play, and at the time of writing the Clippers are legitimate title contenders, but it will always be an open question as to what he and the Raptors could have accomplished had they stayed together.