I’ve tried FastHabit, Life, and Zero.
I like fast habit the best because it’s no frills and is pretty much just a timer. I believe it was free or had a small one time fee.
Zero was overly complicated, but also has a ton of resources and information about the whole fasting process and zones. But it requires a monthly subscription, to use anything but the timer.
I don’t remember much about Life, I didn’t like it. But it has a social aspect to it, so you can fast with your friends.