We have rules here? News to me.

I forgot to give an honorable mention to Vaughan Williams as I don't think he was listed yet--but every movie soundtrack composer should pay homage to Vaughan Willliams' Symphony No. 5 in D Major.

Just 5?!
 

Haydn
Debussy
Mozart
Saint Saens
Beethoven

I think more research and listening may change that list, but as of right now, these are the 5 I appreciate the most! I am a big fan of the last four because of the piano duets they wrote. When almost everything you play is a solo, it's great to collaborate with other pianists and make something beautiful as an ensemble.
(Haydn's my number one right now because I'm learning one of his sonatas, and it is a great learning experience!)

1. David Maslanka
2. Ravel
3. Caroline Lizotte
4. Marcel Tournier
5. Lutoslawski

I might be slightly bias toward harpists :)

Scriabin
Bach
Fauré
Hildergard of Bingen
Scarlatti

Antonín Dvořák
Johannes Brahms
Modest Mussorgsky
Richard Wagner
Pyotr Ilyich Tchaikovsky

(In case you can't tell Romantic is my favorite period.)

Johann Sebastian Bach
Wolfgang Amadeus Mozart
Ludwig von Beethoven
Sergei Rachmaninoff
Frederic Chopin

One of my favorite pieces by Bach:

I always had trouble with my multiplications in school. Schoolhouse Rock helped me a great deal. This song isn't just educational, it also sounds pretty cool. But I have to admit, not that this is a bad thing, because it's not, but I'm still waiting for Tim Burton to use this song in one of his movies.....

A great piece by Sergei Rachmaninoff:

Best 5 Composers?
 

The finest composers aren't necessarily the ones I would like. The best are: -

5 - Shostakovich 15 astonishing Symphonies and 15 string quartets which invigorated the form, 2 great operas, a string quintet and so on showing a huge range of form and function - looking both forwards and backward. Unsurpassed irony and wit.

4 - Bartok - trail-blazed incorporation of South-East European, African and Asian musics into Austro-German mainstream. Master of counterpoint (rivaling to Bach), surpasses Brahms and Busoni in close harmony and extended and intense contrapuntal constructs while retaining a symphonic expansiveness in non-symphonic works, e.g. the scherzo movements in the 4th and 5th string quartets sound like a full orchestra is playing.

3 - Bach - Remarkable exploration of harmony and counterpoint as servants of pure beauty. Capable of conveying intense emotion within extended dramatic forms - Matthew Passion and Mass in B minor may be the finest large scale works in the repertoire, the ricecare and fugue from a musical offering the most transcendental of any Western music.

2 - Schubert's musical attributes combine seamlessly into something beyond the sublime. Divine lyricism, impeccable dramatic structure and romantic vision. NOTHING in Mozart remotely matches the spiritual journey of the late C major Quintet, the melancholy wit of the Arpeggione sonata, the symphonic integrity of the last three piano sonata, the depths of human feeling of the unfinished. I am weeping as I write.

1 - Beethoven displays every single attribute discussed above, with the possible exception of pulling in other musical cultures, although he was always interested. Bach and Schubert maybe divine witnesses of the human condition, Beethoven actually shapes it. In the third symphony we see the composer represent and confront his personal circumstances. We see his individuality rise triumphantly in musical form. In the fifth real demons are represented and confronted and vanquished in so complete a way that it remains a standard of triumph the human spirit. The sixth introduces the world to tone painting. The Allegretto of 7th has the most curious delineation of form and the wildest fugatos known to man. The late B flat quartet takes us through a remarkable and varied transcendental landscapes and ends with an exploration of atonality nearly a 100 years before Schoenberg. The late A minor contains the Heiliger Dankgesang eines Genesenen an die Gottheit, in der lydischen Tonart" (Holy song of thanksgiving of a convalescent to the Deity, in the Lydian Mode) the most profound meeting of human being and the divine in ALL human creation. I haven't even mentioned the ninth symphony of the Missa Solemnis.