It depends on your level and on your taste...
For references use the search feature...
Here is what I think, and there is of course much much more that is very good litterature.
Bach: Most things are interresting... Notebook for Anna Magdalena , 24 preludes are for starters... then you have French and English suites, then Partitas, then 48 preludes and fuges , and the Goldberg variations...
Beethoven: 32 sonatas
Chopin: Everything

Waltzes, Mazurkas, Nocturnes, Ballades, Etudes, Preludes, Impromptus, Polonaises, etc ... Etudes are very good for technique, but pretty high level, they are also fantastic musical pieces.
Debussy: Preludes
Mozart: Sonatas
Rachmaninov: Preludes
Scarlatti: Sonatas
I think Bach 48 preludes and fuges + Chopin 24 etudes + Beethoven 32 sonatas + Debussy preludes book 1 + Rachmaninov Preludes is a very very nice repertory

(Although I dont think anyone plays only these things...)
The piano litterature is so rich that you can almost drown in it

You will never run out of pieces to learn
