A whole industry has grown up to help answer that question. As a starting point I recommend downloading the report:
It has a section on app monetization.
For games, free to download plus an addictive or social or quest oriented format works well as you can monetise with in app purchase or subscription from repeat customers.
For paid downloads, a strong brand AAA title or cross-platform leverage using a brand or established game present elsewhere is a good approach.
To bridge the two extremes, a common technique is via a retro-games approach because capital requirements are low (due to simplistic graphics) and modelling old favorites is low-risk as that is based on tried-and-proved addictive game models (e.g. crossing the road games, pac man games, etc.)
There is more to discuss on it. Look me up on linked in if you want to find out more or sound out any ideas.