Mozart Piano Concerto 21.

This is not an easy question to answer. It is so subjective. Besides the "quality" of the performance and the sonic quality of the recording, there is the matter of modern vs. "original" instruments.

Many of the recordings mentioned above are on modern instruments with modern-sized orchestras. To that list, I would add:

Maria João Pires. Chamber Orchestra of Europe, cond. Claudio Abbado (DG, 1995)

For recordings using "original" instruments, etc., the definitive recordings (in my opinion) are by Malcolm Bilson, who has issues the entire cycle on CD. For No. 21, see:

Malcolm Bilson. English Baroque Soloists, cond. Gardiner (Archiv, 1987).

The Bilson "original instrument performance on Archive is excellent. I have his whole set and it is a must listen for anyone interested in Mozart.

Brendel/Mackerras/Scottish Orchestra, perform spectatular concerti. Macherras' conducting is outstanding!! Don't think they recorded #21. But check out #25, my favorite. I perfer these recordings vs the Uchida/Tate/English Orchestra.

Well i prefer Uchida over Brendel, but prefer Mackerras vs Tate. Brendel's playing lacks a gusto that is necessary for Mozart. Even Uchida takes it alittle too easy on the keyboard. Mackerras' conducting of Mozart with the Scottish C. O. is spectactular. As well Mackerras conducts the Prague C. O. with genius on the Symphonies. Though i like Walter/Columbia S. O. , the Mackerras are essential to own.

Mikhail Pletnev is tackling the Mozart repetoire. So far, he's done 9, 20, 23, 24. His pianism is stunning and his grace is astonishing.