If you were to learn and entire collection of pieces....

[j_menz]

This is where Scriabin fills in for Rachmaninoff
 

Scriabin takes the same approach regarding the relationship between melodies and harmony as that taken by Bach.  It is for good reason that the best interpreters of Scriabin are often also noted Bach performers.

[thorn]

Debussy Images I,II (I play 5/6, for some reason I can't ever get the hang of Mouvement)

That's kind of cheating though... if we're talking virtually from scratch, the Debussy and Ligeti Etudes, Messiaen Vingt Regards and Catalogue d'Oiseaux.

But to be honest the most I can probably hope to play well would be a couple from each collection. If I was really pushed to pick one set to study in depth in its entirety, I'd probably go for the Vingt Regards.

[jollisg]

Chopin's ballades!

[mussorgsky]

Debussy preludes.

[stoudemirestat]

Liszt Annees de Pelerinage.

[quantum]

Scriabin Sonatas.  

This is where Scriabin fills in for Rachmaninoff
 

This is where Rachmaninoff praises Medtner.  Medtner's compositional discipline can sure be appreciated more with the study of Bach.   

[patrickd]

Medtner's op 51, or Mussorgsky's pictures.

[danhuyle]

My favorite sets

Albeniz - Iberia
Liszt - Transcendental Etudes, Paganini Etudes, Ballades, Sonata
Chopin - Ballades, Scherzi, Fantasia Op49
Scriabin - Etudes Op8, 2 Poemes Op32, Fantasia Op28
Ravel - Gaspard De La Nuit
Tan Dun - 8 memories in Watercolor