Conventional wisdom seems to direct us towards Haydn & earlyish Mozart, but judging by the repetoire you are working on, you have a lot more to choose from.Personally, I would almost certainly start with the Stamitz F Major which is rarely heard and contemporary with Mozart, or perhaps the delightful Marlborough Variations by Vogler from the same era. For the Romantics, I have heard Grieg and Saint Saens 2 mentioned several times here, but Martucci 2, Sterndale Bennett 4 or Stanford 2 make interesting alternatives and excessive demands on the pianist are not present.There are too many "hardest" topics here so, I will only say that the Schytte is a bastard.Thal
The Kabalevsky Piano concerto is wonderful. There are a lot of opportunities for very colorful playing that most performers seem to miss. I recently accompanied a student playing this concerto at a competition, so I suppose it springs to mind only because I was involved with it recently.