1. Jalen Brunson
The Dallas Mavericks' most devastating loss in free agency over the last few years was Jalen Brunson leaving Dallas and signing with the New York Knicks.
This seemed like an expected move leading up to the decision, and Brunson's number seemed a bit high now, but Brunson has already outplayed his contract in New York after one season.
In his first season with the Knicks, Brunson averaged 24.0 points, 3.5 rebounds, 6.2 assists, and 0.9 steals per game. Brunson had a massive jump in performance last season and has cemented himself as a top-10 point guard in the NBA.
Dallas has the better player now, as they traded for Kyrie Irving and re-signed him this summer, but keeping Brunson would have been a great move. His contract would have been much smaller than the deal they signed Irving to, and Brunson's career trajectory is still on the way up.
It seems like he'll continue to be one of the best point guards in the NBA for the foreseeable future, and Dallas is likely regretting coughing up to keep him a Maverick. It's unknown if that was even possible, as some people think that Brunson was heading to the Knicks all along, but Dallas had the chance to extend him multiple times and passed.