Tommy Edman or Brendan Donovan
Assuming Nolan Gorman takes the role as the primary second baseman long-term, or even at least someone who plays there a lot, and Masyn Winn becomes the shortstop of the future here soon, it gets hard to find clear roles for both Edman and Donovan on the club, especially if they are both making more than contract renewal or arbitration money.
Both players provide the Cardinals with awesome defense at multiple positions and do so while bringing their own unique skill sets to the plate as well. Neither player is a superstar, but both are the essential "glue guys" that you need on your team to have success.
Personally, I think Brendan Donovan may be the better extension candidate while Edman may be an extremely valuable trade chip in the future. Again, this is assuming Winn pans out, but Edman is a few years closer to free agency than Donovan, likely requiring more dollars to extend him. Donovan also seems like a more natural fit for the true utility man role, while Edman thrives best in the middle infield. Lastly, I think Edman could really help shore up another area of their roster in a trade, which is valuable if they truly have this elite middle infield depth.
Extending either player would be great for the organization though, but I would prioritize the first three players on this list before any more here.