Flutter is the second nicest platform I've found for developing for Android (losing to Jetpack Compose) and best one for cross-platform. As a user though, I hate using Flutter (and non-native) apps for simple things. They often look totally different from the rest of the OS. For products that exist as something other than apps, it's fine, but I still prefer native Material You apps.

The best Android app for a product I've used is the AnonAddy app. It's stylish Material You while feeling very custom and tailored.