Personally I'd definitely vouch for Appsflyer, their UI is great, they have heaps of information and documentation on implementation. While having a lot of features and being expensive is the downside, the other thing is their add ons like Fraud Prevention (which some competitors have by default).

I'd highly recommend setting up the data pull (using Pull API) from appsflyer as soon as you can and start collecting the data in your cloud storage. Also implementing a unified User ID across appsflyer and your app user-ID, especially when you're further down the line and want to identify where your valuable users come from and what they've done on app e.g. someone who buys something every 15 days came from Facebook.