Health
The Health app is gaining new machine learning support that will provide new summary and highlight views to help surface important information in the app, based on what it thinks will be most relevant and interesting to the user at any particular time.
Apple Watch users are also gaining new hearing monitoring features and a cycle tracking app for women, which will of course sync to the Health app in iOS 13. In the case of the Cycles app, however, this will also be available natively on iOS 13 without the need for an Apple Watch.