Hi Jaoquim, I would go for
1 - GA (free) +
2 - 1 out of the mobile marketing tracking family (like mobileapptracking which used to be good, not so much anymore after the Facebook fiasco, or apsalar, kochava, localytics etc) +
3 - mixpanel (optional, to track in-app events segmented by source..+ more reports)
4 - Crashlytics
atleast to start with.
If you can find a really good tool for mobile marketing tracking, that can give you Mixpanel like reports, I would even skip Mixpanel. Really depends on your priorities like pricing/requirements/reports you need most etc.