I don't know if anyone remembers my smash hit PocketPC music app PhoenixStudio from 2001, but I ported it to Android, just because. An iOS port may be coming later.
PhoenixStudio is basically a clone of Propellerhead's Rebirth in capabilities, but with a different UI and some enhancements here and there. 3 synths, a drum machine and a full sequencer with automation.
As the interface was originally designed for stylus control, you're gonna need a Galaxy Note (or simply a device with a really large screen, or very small fingers).
It's free to download, save functionality can be purchased in-app but the rest is all free.