My favourite part about Firebase is the sheer range of available features - the toolkit appears to include a tool for everything. The way in which the tools integrate with each other seamlessly allows for a rapid workflow which is ridiculously easy to get started with, and the pay-as-you-go billing means that adding new tools is super easy without having to mess around with payment.
Furthermore, the integration with other Google products, such as Google Cloud Platform, AdSense and even things as simple as OAuth make the experience a breeze and allow you to easily add features to your product you hadn't even considered previously (such as machine learning, or content translation - both just a click away).
Also, it would be stupid not to pick up on the actual pricing - the prices are so low, and free tiers so generous, that for most applications there is almost no need to even add billing in the first place. Review collected by and hosted on G2.com.