Lots of my recommendations are already here...
Bedrich Smetana:
Ma Vlast, especially the "Vltava" part - which is also the most famous part, since the theme is very similar to the Israeli national hymn - the hymn may or may not have been inspired by "Vltava", or they're both drawing from older music. "Värmlandsvisan", which Kris might know, has another very similar theme.
Sergei Rachmaninov:
2nd and
3rd Piano Concerto - I can only agree with tonymiro's admiration for Martha Argerich's interpretation of the 3rd, although sometimes I'm more in the mood for a more transparent version, like Stephen Hough. Also his
etudes tableaux - fiendishly difficult, some of them. And
Isle of the Dead - very sombre and atmospheric - as well as
Symphonic dances. And be sure to have a listen to
Rhapsody on a Theme by Paganini. 24 variations on Paganini's 24th caprice. Lots of very impish humor in there. Everything by Rach is great

The Rhapsody and the 3rd concerto are among my favorite pieces to play on the piano - out of the entire repertoire. Those, and:
Sergei Prokofiev:
3rd Piano Concerto - wonderfully cacaphonic, and yet after a few listens, everything in it makes sense. A very classical piece, with very non-classical harmonies/chords. Also check the music for
Romeo and Juliet (ballet, but also collected as suites).
Felix Mendelssohn:
Violin Concerto - really the second, but I've met few who've ever heard of his first

Wonderful themes, and a great cadenza (solo) for the violin, which I'm sure could inspire some guitar riffs
Fryderyk Chopin:
Etudes - all 24 of them (two sets of 12) - much more than study pieces. Used one of them (op. 10 no. 1) as example of arpeggios-from-hell-arpeggiating for piano in a thread on the theory board

Although op. 10 no. 4 is much more fun to play. Op. 10 no. 5, the right hand is using only the black keys - effectively making the G-flat major pentatonic scale

. Anything by Chopin, really -
preludes,
waltzes,
nocturnes, all of the piano solo pieces (not really as interested in his concertos).
Igor Stravinsky: Other than
Rite of Spring, also have a listen to
The Firebird - more ear-pleasing, but just as primal.
Henryk Gorecki:
3rd symphony (Symphony of Sorrowful Songs). Very very moving, when it's done right. Naxos, being a budget label (also available on iTunes) has probably the best recording of it I've ever heard.
Wolfgang Amadeus Mozart: I mostly dislike Mozart, except for all of his vocal work - his masses, including the
Requiem, as well as his operas. Wonderful creativity and imagination in those. Not so much in his orchestral and piano works, I find.
Ludwig van Beethoven: His most famous piano sonatas - I much prefer
Pathetique and
Appasionata to Moonlight.
Symphonies - the 3rd, the 7th and the 9th are my favorites.
Arvo Pärt:
Fratres,
Summa and
Tabula Rasa especially. Modern composer, medieval inspiration
Franz Liszt:
Hungarian Rhapsodies (no. 2 is the most famous one, but others are worth of a listen too),
Mephisto Waltz No. 1, his
Dante and Faust symphonies, and his
piano sonata. Also check out his piano transcription of
Saint-Säens' "Danse Macabre" - preferably also listen to the original orchestral piece.
Witold Lutoslawski: ... whom tonymiro already mentioned, also did his own set of variations on
Paganini's caprice no. 24.
Bela Bartok:
Concerto for Orchestra and
Music for Strings, Percussion and Celeste. Lots of interesting rhythmic and sonic ideas there.
Edvard Grieg:
Piano Concerto and music from
Peer Gynt. Very Norwegian folk music-inspired. Liszt admired the concerto a lot (although reportedly, he managed to play it (too fast) straight from the sheet when first seeing it

)
Claude Debussy:
Preludes for piano.
La Mer. People tend to call it the musical version of the impressionism in pictorial art - very "floaty", much of Debussy's work

...
I could go on and on

The places where I've listed the same as others... Take that to mean "essential"