Figure is probably the iOS music creation app that gets the most use on my phone, very simple to use and even my 2 year old can crank out some decent beats in just a few minutes. It has been free only once before and is usually US$0.99 , from the wording of the latest update it looks like it may now be free permanently so not sure if this is the best place to post this or not.
I can't recommend this app enough if you are even slightly into music creation apps.
Edit: Here's the official video for it to give an idea of its usage. Its constantly being updated with new sounds and features so has evolved a bit since this video was made. https://youtu.be/gLLjRH6GJec
Looks cool, thanks!