2015: Kevon Looney over Nikola Milutinov with the 26th pick
I've been on record saying that the Spurs' decision to select center Nikola Milutinov 26th in 2015 was their worst draft decision. It probably still is, especially with the players that were still on the board, including Golden State Warriors center Kevon Looney. Looney had a slow start to his career but has really come into his own over the last couple of seasons, especially in this year's playoffs. Looney showed why he is one of the best offensive rebounders by being a terror on the glass, averaging 4.7 offensive rebounds per game and 13.2 overall in just 25 minutes per game.
Getting a starter with a late first-round pick is something the Spurs are known for, but instead of nabbing Looney, they chose to draft and stash Milutinov, who ultimately never played a single game with the team or in the NBA. Swap Milutinov out for Looney, and the Spurs would have gotten a lot more out of the 26th pick.
Instead, the Spurs relied on veteran players to fill the role of center until Jakob Poeltl eventually became a starter. Now, they have a chance to fill the role for at least a decade with Wembanyama, though it would have been nice to have actually used that 2015 pick to get a player who could contribute.