Mozart Piano Concerto 21.

For #21 you can't go wrong with Murray Perahia. He has recorded it more than once. Try the one with the Chamber Orchestra of Europe with him conducting from the piano. It is on Sony Classical SK46485
(Comes with Concerto #27)

On LP record look for the one on Columbia Records with the English Chamber Orchestra. Album number M34562

Try Andras Schiff and Alfred Brendel!
Good luck!

In addition to the three fine recordings mentioned above,consider the George Szell-Cleveland Orchestra recording and the Academy of St. Martains in the Fields Orchestra with Clifford Cuzon.
My personal favorite is the Cleveland but that's subjective.

I strongly recommend Dinu Lipatti/Lucerne Festival Orch./Karajan (live & fascinating -- mono recording!) and...
Rob. Casadesus/Columbia SO/G. Szell,
both on Sony (the latter a 3cd box comprising 21-24 + 26-27 et alia). My taste also tallies with Sugar's re: Perahia. The Perahia version I particularly like is with the English Chamber Orch, part of a recording of the complete piano concerti.

I will ditto the Curzon recording. Curzon is my favorite Mozartian pianist. But the recording I have is with Kubelik and the Barvarian Radio Symphony. I did not recommended it figuring the Perahia with modern CD sound would appeal more to audiophiles. I am a music lover first and I care about the performance 99%. The high end audio just makes it sound as good as possible.

For Schiff I hope it is newer recording. I never thought Schiff had "got it" as far as Mozart when he was young. I found his early attempts at Mozart competent but unremarkable, including the time I heard him at Tanglewood years ago. I do find as he has matured, his Mozart has matured as well!