Pianistic: Chopin, Beethoven, Liszt, Schumann, Henselt, Schubert
Their style of composition makes excellent use of the typical qualities of the piano as an instrument. People who say Schumann or Chopin are unpianistic composers either are just looking from one particular perspective (like Hamelin or Brendel) or are uncapable of playing their more taxing works. Someone came up with Chopin's op. 9-1 as an example of how unpianistic he is supposed to be. That piece is very comfortable to play and also gets orchestral sounds out of the piano if well played.
Schumann wrote a fair number of extremely difficult works, but when I started learning 'Carnaval' I noticed that seemingly uncomfortable passages are in fact quite ergonomical if you use the correct fingering and practice slowly. OK, there's stuff that seems to be taxing to even the best pianists, like the Toccata, the Symphonic Etudes, or the Paganini Etudes op. 10, but these are meant to strengthen technique, so that still makes sense from a pianistic perspective.
Unpianistic: Bach, Alkan, Berwald
Bach's a very effective keyboard composer but his style of writing is not the most efficient in making use of the sound one can get from a modern piano. Which is not surprising, since he wrote them for the organ or the cembalo. That's not just my opinion, it was a widely held belief that spawned hundreds of arrangements of his works, by the likes of Busoni, Tausig, and countless others.
Alkan was, according to Alkan-scholar Raymond Lewenthal, really a frustrated symphonic composer, who wrote an enormous amount of extremely difficult works that are extremely uncomfortable to play even to seasoned concert pianists. They are so difficult that it takes an enormous amount of time to properly learn them, which has led to people who championed his music, like Busoni, not to learn too many of them because they just didn;t find the time.
In other words, his music probably would have served the classical music scene better, as in, reached out to a bigger audience, had it been orchestral, due to it having been more widely playable with an orchestra instead of just a couple of 100 virtuoso pianists worldwide.
Regarding Berwald, take for example the Fantasy on 2 Swedish Folk Melodies. I find the style very Lisztian (it is also from the same time, 1850s), but the execution is not making as efficient use of the sound one can get from the piano as they could have had. Especially because their left hand arrangement is not as effective as Liszt's would have been, often eschewing large leaps and huge (but comfortably playable) arpeggios, therefore using a narrow pitch range in one measure. That hampers the richness of the tonal palette, something Liszt, Schumann, or Chopin would never have done.