I'll go ahead and make my top 5 list. To make up for that, I'll explain my choices.
1)Beethoven-This one is by a wide margin. He not only composed a great amount of material, but most of it's of high quality. Not to mention that he's a huge figure in the development of music. The 9th Symphony, alone, influenced more music than many other composers did with all of their works. Most importantly he wrote great music. The 5th, 6th, 7th, and 9th Symphonies, the Kreutzer Sonata, the 3rd and 5th Cello Sonatas, the Waldstein, Pathetique, op 101, op111, etc...
2)Mozart-He seems to be somewhat unpopular in this forum, but I love him, even his happy stuff (there's a place in music for happiness.) His Opera's are the best I've ever heard. If he'd have lived longer, he might have even been #1.
3)Mahler-This one might surprise some people, but I can defend my decision to stick him here based off of one work, the Resurection Symphony. That may be my favorite piece ever written. His symphonies represent the peak of Romanticism, and his Lieder are incredibly moving.
4)Bach-He seems to be nearly worshiped at this forum, but I disagree. Many of his religuous works and heavily contrapuntal works are kinda boring. However, in works like the Goldberg Variations, Violin Concertos, Brandenburg Concertos, and Italian concerto he writes brilliantly. Also he wasn't paticularly revolutionary. When he composed, he was falling behind the times musically. *puts on flame-retardant vest*
5)Mendelssohn-He represents the perfect balance of Classicism and Romanticism. The G minor Piano Concerto and the E minor Violin Concerto, are both contenders for the best in their genres.