I am a big fan of the Zero app, which is a tracker for intermittent fasting. It is free, attractive, and has just enough features to keep me engaged and motivated, and no more.
Currently, my favorite feature is its Apple Watch complication, which fills up a ring (similar to the Activity app’s rings) as you progress through your fasting period. I find that filling that ring every day, and the app's cheerful notifications every morning, really help motivate me to stop snacking or having an extra meal at night.
The only thing I don't like about it is that it requires you to set up an account with an email address. While the company behind it does not appear to be abusing my privacy in any way, I would prefer that they didn’t require an account. Perhaps they will adopt Sign in with Apple when it becomes available this fall.
If you are interested in Zero, download it from the iOS App Store.