GCP
I have mostly worked with GCP on my own. It is the little brother of the
Firebase
Firebase and Flutter is a very cool combo. It has great material on how to work with and helpful docs. It makes creating a simple crud app in Flutter super easy. Firebase has authentication, databases, cloud functions, and a lot of features that make it an ideal service for projects.
Firebase, however, is a property service and it will be a pain to migrate out. But Google said it wont be evil.