An impossible choice to make, given the consistenly high quality of Medtner's work, but the Night Wind Sonata and the two sonatas Op.53 deserve places near the top of such a list - also his second sonata for violin and piano as well as his third piano concerto whose haunting opening is magical. Much the same could be said of some of his songs...
Medtner's 3rd Concerto is probably my favorite piano concerto
I was asked to write a cadenza for it many years ago...
Interesting! Did that ever come into fruition? Also, where in the concerto would it have gone/did it go?
Op.51 No.3 (it's graceful character and the musical portrayal of Cinderella and the trying the glass slipper,
Fairy Tale in B Minor, op. 20, no. 2