There are some I enjoy (mostly fairly well known composers)
Berio- Sequenza IV, or Cinque Variazioni
Berg- Sonata op.1
Bolcom- Etudes, New Etudes
Boulez- Douze Notations, Piano Sonata no.1, Incises
Carter- Piano Sonata
Donatoni- Rima
Ligeti- Etudes
Rzewski- North American Ballades, Four Pieces, Squares, Variations on "The people united will never be defeated" (or selection of the huge set of variations)
Schoenberg- op.11, op.19, op.23
Stockhausen- Klavierstucke (I personally like VI, VII, VIII or IX the most, but they are all nice)
Ustvolskaya- Any of the sonatas
Claude Vivier- Shiraz
A few suggestions to consider.