Minnesota Twins pitcher Sonny Gray
This one should be a no-brainer. If you see what Gray has accomplished this season with Minnesota, I think the Cardinals would be unwise to not go after him this winter.
Gray has very quietly been one of the best starters in the league this season, he only has 8 wins due to a lack of run support, but a 2.79 ERA and 184 innings pitched is his best since 2015. His 2.77 FIP (Fielded Independent Pitching) is the lowest in baseball, which basically means he was the best at limiting what he could control (walks, hit by pitches, home runs, etc.) and he only allowed 8 home runs in 32 starts, his 0.4 HR/9 is easily the lowest in the league.
I wouldn't commit to Gray for too long, as he will turn 34 next month, but a 2-3 year deal would give the Cardinals an All-Star pitcher manning their rotation. Since his time with the Yankees ended, Gray has shown that he pitches better in a more calming environment where there's less "pressure" to pitch in. Also, his stuff would work well pitching in Busch Stadium, a ballpark that is difficult to hit home runs in, Gray could be a perfect fit.