2011: Isaiah Thomas over Adam Hanga with the 59th pick
While selecting both Leonard and Butler would've made for a great draft, the Spurs also had a second-round pick that year that could have been better used. With the 59th pick in the draft, the Spurs selected Adam Hanga, an athletic wing known for his defensive ability. Unfortunately, Hanga never ended up joining the team, while Isaiah Thomas, who was taken last in the draft, managed to carve out quite a career for himself.
The last pick is nicknamed "Mr. Irrelevant" for a reason, since few, if any, ever even make the NBA. Of course, Thomas did. He made two all-star teams, including during the 2016–17 season, in which he averaged 29 points a game and finished 5th in MVP voting. That would obviously have made him a better choice for the Spurs, even though he has largely been a journeyman in the six years since a serious hip injury.
Ironically, the second half of his career was similar to that of most second-round picks. Still, Thomas' peak is better than most players', making him an ideal inclusion on this hypothetical Spurs dream team, joining two yet-to-be-mentioned players at point guard.