Hi mrs.s.overall,
Great question! There are a number of migraine diary apps out there, it really depends on personal preference, in my opinion.
Here's a short list, not in any particular order for apps you can look into;
Migraine Buddy
Migraine Monitor
N-1 app
I hope that helps. Will you let us know what you think of them?
Wishing you a low pain day,
Nancy