Jasuto

This app let's you create your own synth sounds from the ground up and it's actually really easy to use once you get the hang of it. It's really the only way to come up with unique sounds, then you can paste them into your favorite synth app. But it also has a step sequencer and keyboard included so you can create loops instead, as well as automation. This app really covers a lot of ground and does things a normal synth can only dream of.