Hmmm, that is a way hard question. I am a huge fan of so many things. I have a whole lot of favorites for different eras or genres and for different reasons. I like several types of compositions and techniques and ways of expressing and I think a lot of different composers do those things I like at different times. I am not an incredibly huge fan of the old clean and cut traditional sounding composition. That is to say that when Bach or whomever writes a piece they write with so many rules and such structure that it doesn't feel as natural and as free and expressive to me as later music got to be. That is not to say that I cannot appreciate things that occur in that music or that I don't realize the brilliance of their art for the time that it was composed. For instance, though I am not an inordinately strong fan of the Baroque or Classical eras a few of my top favorite pieces come from those eras. I am extremely impressed with Bach's Goldberg Variations and I absolutely love listening to them played. I am very fond of Bach's Jesu Meine Fruede. I really love parts of Handel's Messiah (especially Worthy is the Lamb). Bach's Suite for solo Cello stands out to me as an iconic piece for great reasons and I acknowledge the reasons why. Albinoni's Adagio for strings and Organ is a standout piece of music in all of music history. I give credit to Mozart for his Requiem Mass. On the whole I think Mozart is one of the most overrated composers of all time. I understand that his music served a purpose and it pushed the succession of ideas along in classical music in a chronological sense. However, I don't find most of it to be that creative or fulfilling. I do love me some Beethoven. I have no love to show for his piano concertos but just about everything else gets me going. He is the king of the symphony. His piano sonatas are great. I don't care for the operas, but the Overture to Egmont is great music.
All that being said, my thing, the stuff that gets me fired up, is when music became about emotion and expression to an almost over the top grandeur. There is a pocket of things that sing to me - that make life worth living. You could possibly split the stuff I love into two categories - still classically structured and not (the modern stuff). We would be talking anything from Greig to Beethoven to Tchaikovsky and on the other side anything from Barber to Messiaen and Bernstein and Stravinsky and John Adams. Then there was a time that came that pushed it too far and I have nothing positive to say about that whole lot of stuff. I would consider all that to be garbage. What I would consider "that" to be would be anything like John Cage, all the later Schoenberg, I find Charles Ives' music to be inane and fruitless, Boulez, Legeti, Stockhausen, Xenakis, Penderecki and even Crumb, John Coolidge Adams, Christopher Rousse. I don't like any of that at all. In general I sum it up as noise. My favorite later, but still somewhat classically structured composers are probably Camille Saint-Saens, Edward Grieg, Sergei Rachmaninoff, Tchaikovsky, Dvorak, Verdi, Puccini. I am also really fond of the french Impressionists Debussy, Ravel, and even Satie. Scriabin just floats out there in his own realm - he is on my top 5 favorite composers list. Then you go into possibly what is my top favorite area and that is your Samuel Barber, Aaron Copland, Leonard Bernstein, George Gershwin. I really like the late romantic/contemporary American stuff. I love me some Prokofiev. Messiaen is extremely interesting to me. If the one previously mentioned isn't my favorite genre or area in music then this is - I am hugely fond of this new stuff. It is basically anything that has to do with the Atlanta School of Composers. I love Theofanidis, Micahel Gandolfi, but especially Adam Schoenberg and Jennifer Higdon. Also, John Adams - he is not one of that school, but he is incredible.
So, all of that being said I would have to take into consideration my type of style and the entirety of a composers life work. When I do that there are a few composers that pop out to me. Probably on more days than not I would give my top spot to Samuel Barber. Some days I would go with John Adams and some with Adam Schoenberg. Though, most of the time I would probably feel Samuel Barber out the most. This is still hard to say because so many of my top moments in musical history are from a varied group of composers. I think if I were to have to list off what I would consider to be the great compositions - the greatest moments in music- for me it would have to include Aaron Copland's 3rd Symphony, Aaron Copland's El Salon Mexico, Scriabin's Poem of Ecstasy, Scriabin's Etude in D sharp Minor Op. 8/12 (to sneak one of my all time favorite piano pieces in the list), Aram Khachaturian's Violin Concerto (especially the second but also the third movements), Bela Bartok's Miraculous Mandarin, Camille Saint-Saens Organ Symphony, Saint-Saens 2nd Piano Concerto, Theofanidis's Rainbow Body, Debussy's La Mer, Grieg's Piano concerto in A Minor, Andante Religioso from Grieg's Holberg Suite, the whole last movement or two of Mahler's 2nd symphony, The Finale to Stravinsky's Firebird Suite, the Dharma at Big Sur by John Adams, Bernstein's Chichester Psalms, Bernstein's On the Waterfront, Marjan Mozetich's Passion of Angels Harp Concerto, the Interlude from Cavelleria Rusticana by Mascagni, Ravel's Daphnis et Chloe, Ravel's Jeux D'eau to throw in another favorite piano piece, also Ravel's Pavane pour une infante definite, Michael gandolfi's Garden of Cosmic Speculation (especially the movement entitled The Quark Walk), the very last movement of Messiaen's Quartet for the End of Time, the finale from The Pines of Rome by Respighi, Tchaikovsky's 1812 Overture, Tchaikosvy's March Slave, Rachmaninoff's Isle of the Dead, Rachmaninoff's entire 2nd piano concerto, Samuel Barber's Symphony 1, Samuel Barber's 2nd Essay for Orchestra, Samuel Barber's Adagio for Strings, Barber's Medea's Meditation and Dance of Vengeance, Finale from Prokofiev's Cinderella Ballet, Procession of the Fallen and Judgement of the Prisoners and the final chorus from Prokofiev's Alexander Nevsky, Final to Prokofiev's Ivan the Terrible score, Messiaen's Turangalila Symphony, John Adam's Short Ride in a Fast Machine, John Adam's Lollapalooza, John Adam's Harmonium, John Adam's Chairman Dances, Jennifer Higdon's On a Wire, Adam Schoenberg's Finding Rothko, Adam Schoenberg's American Symphony, Adam Schoenberg's Picture Studies (especially the kandinsky portion), Adam Schoenberg's La Luna Azul.
There are probably a few pieces I missed out on, but that is about all of what will get me going the best. Obviously, I am a huge Prokofiev, John Adams, Adam Schoenberg, Samuel Barber, etc. fan. Though I think the simple answer to your question is Samuel Barber 6 out of 7 days.