A Final Fantasy 6 town theme, 'Kids Run around the Corner' or something stupid like that. That was the first music I ever arranged for anything. Then came a slew of video game MIDIs.
Scott Joplin's Maple Leaf Rag was probably my biggest influence - it's the first piece I could play on piano. It's also why Aflat is my favorite key.
I also learned a great deal about form and structure from playing all that ragtime. Oh, and syncopation. Ragtime rocks!!
And then there's Nixon in China, John Adams' opera from 1987. I discovered this piece late in my 'minimalism' stage, and practically changed how I wrote music. John Adams is the man.