Looptastic Producer by Sound Trends LLC.
A lot of people use this for creating songs with pre-made loops or DJ'ing live but it's also a great teaching tool to demonstrate how different genres of music are put together. It already separates the percussion, bass, main melody, and vox for you. It's a really fun app.
Bebot - Robot Synth by Normalware
This really has a good tone variety with a number of different effects to choose from. There's no need to know anything about music to make awesome melodies because it already chooses the correct notes for you. Just choose a scale to work with and you're set. Try using more than one finger to slide across the screen in harmony.
And the number one music app of choice was "Other." With all the new music apps in development, apparently 16 choices didn't cover enough ground. Share what your favorite unlisted app was in the comments section below.