Starting pitcher Joe Musgrove
Musgrove was the key piece of this trade, and while he wasn't overly dominant with the Pirates, it was mostly due to the pitching coach and defense. While he showed so much potential, he was never able to show his dominance with the Pirates.
After getting traded to the Padres, he threw two complete game shutouts, including the first no-hitter in Padres history. Since getting traded, he signed a 5-year/$100 million contract with them, turning his career around and becoming an All-Star in 2022, showing hitters why he was once a top prospect and a key trade piece in the Cole trade.
While the Musgrove trade may prove to be a vital trade in helping to get the Pirates back to competitiveness, that's a story for another day. The Mets received Joey Lucchesi from the Padres, the Pirates received Omar Cruz, Drake Fellows, Hudson Head, and David Bednar from the Padres, and the Mets traded Endy Rodriguez to the Pirates.