Schubert's Impromptus are among my most favorite piano music. Schubert is unique among composers in that he can evoke the most complex emotions out of the simplest and most irresistible melodies, and the changes of mood even within the space of a few bars are staggering in these pieces. To me they are the greatest measure of a pianist's heart, because while the technical challenges of the Impromptus are not enormous, a pianist must possess the kind of empathy necessary to follow the music's journey into the depths of human heartbreak and joy.
Because I love this music so much, I either own or have borrowed from libraries many interpretations--Perahia, Uchida, Zimerman, Pires, Kempff--yet Brendel remains the standard against which all the others must be measured. His approach to the Impromptus is deceptively direct, never overwrought and precious, and he just lays bare their infinite richness without holding back or muddying their overall effect.
My other favorite is Perahia, who is a little more restrained in his playing, and who is in his own way every bit as impressive as Brendel, so I'd have to say it is difficult to choose between the two. In the end I suppose it just comes down to personal preference, and here I have to go with Brendel, because his interpretations seem a shade less mannered and communicate to me the improvisatory (ie. the impromptu) nature of these compositions. Better though to own both, and hear for yourself the kinds of divergent responses these masterpieces cvan evoke.