I can't narrow it down to one favorite, so I'll list several favorites:
Bach: Brandenburg 3, 6 Suites for Unaccompanied Cello
Bartók: Six Romanian Dances
Beethoven: 1st, 2nd, 4th, 5th and 9th symphonies, "Tempest", "Pathétique", and "Mondschein" piano sonatas, and basically anything else he ever wrote or considered writing.
Borodin: 4th movement of 2nd symphony
Brahms: 4th movement of 1st symphony, Hungarian Dances Nos. 2 and 5
Delibes: Sylvia ballet
Dvorak: 8th symphony
Grieg: A minor piano concerto
Handel: Water Music and Fireworks Music suites, Arrival of the Queen of Sheba
Holst: Mercury, Venus, Mars and Jupiter from The Planets
Mahler: 1st, 4th, 5th, 6th, 8th and 9th symphonies
Mendelssohn: 4th movement of 5th symphony
Mozart: 25th, 29th, 39th, 40th and 41st symphonies, overture to Die Zauberflöte, Requiem (especially Introitus, Kyrie and Dies Irae)
Mussorgsky: Pictures at an Exhibition (both piano original and Ravel's orchestration), prelude to Khovanshchina (Dawn over the River Moscow)
Prokofiev: Lieutenant Kijé suite
Rachmaninoff: Prelude in C# minor
Ravel: Jeau d'eau
Respighi: The Pines of Rome
Rimsky-Korsakov: Cappriccio Espagnol, 1st and 4th movements of the Tale of Tsar Saltan, 4th movement of the Snow Maiden
Rossini: overtures to William Tell, Barber of Seville, Siege of Corinth, and Othello
Saint-Saëns: Aquarium, Volière, the Swan, and finale from Carnival of the Animals, A minor cello concerto, Danse Macabre, 3rd symphony
Shostakovich: 24 preludes and fugues, 5th, 7th, 10th, 11th, 12th, adn 15th symphonies, 2nd piano concerto
Strauss (Richard): Don Quixote
Stravinsky: Firebird and Rite of Spring
Tchaikovsky: Capriccio Italien, 2nd, 4th, 5th and 6th symphonies
Verdi: overtures to Nabucco and Un Giorno di Regno, Requiem (especially Dies Irae)
Vivaldi: Concerto alla Rustica, Spring and Winter from The Four Seasons