Mozart Piano Concerto 21.

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!

Sugar, I share your reservations with Sciff. There are 1991/92? Schiff/Camerata Academica of Salzburg Mozarteum /Vegh recordings of a number of concerti. Technically sound, they didn't transport me musically (I haven't heard the 21 in this version).

Istonin on Reference Recordings is another alternative to the wonderful suggestions made.