C'mon, really?
I'll give it a try. Messiaen Vingt Regards. Or Rzewski People Variations. (oops, that's two).
Petrouchka, Gaspard, OC.
Where is the poll button when you need it?
Lol see what I mean?
For contemporary (or at least pseudo-contemporary) piano pieces, my votes go to (and they are judged by their 'importance' and simply how much I like them):
Finnissy English Country Tunes
Stockhausen Klavierstuck X
Stockhausen Klavierstuck XIII
Murail Territoires de l'Oubli
Barraque Sonate pour Piano
Xenakis "Herma"- Musique Symbolique
Boulez Troisieme Sonate pour Piano
Bussotti Pour Clavier
Dusapin 7 Etudes pour Piano
Kurtag Jatekok
Gehlhaar Suite for Piano (Fractal Pieces)
Cage Etudes Australes
Cage Sonatas and Interludes for Prepared Piano
Messiaen Vingt Regards sur l'Enfant Jesus
Otte Das Buch der Klange
Hespos -Z...()
Skalkottas 32 Piano Pieces
Rzewski 36 Variations on the Theme "The People United Will Never be Defeated"
Finnissy History of Photography in Sound
Ferneyhough Opus Contra Naturam
Vivier Shiraz
Ligeti Etudes Book I
Boucourechliev Archipel IV
Lachenmann Guero (1970)
B.A. Zimmermann Konfigurazionen
Globokar "Notes"
Scelsi Suite No. 8 "Bot-ba"
Cowell Dynamic Pieces
and a ton more =/ All very important and influential pieces. If we were to extend the scope to outside simply solo piano repertoire, the list would be even longer XP