Recommended and mentioned products
The most powerful, yet lightest weight crash reporting solution for iOS and Android developers. | Crashlytics
Crash logs: You need to understand how much and why your app is crashing. Crashlytics is the biggest exponent for this, and I highly recommend adding this to your workflow.
Mobile analytics: come with questions, leave with answers
User journeys: I love the ability to see the timeline of the events performed by a specific user. It lets you understand exactly how your users are using your app, in which part of the flow do they feel frustrated, etc. This is a great feature. Amplitude has good support for this.