Shelf is the most stable since it's maintained by the Dart team. I'd start with that. Dart Frog is built on top of Shelf so it has more features. It's made by a well established company so it's also stable. But the API still seems to be under development. Serverpod is set to be very powerful and do everything. As long as they can find a good long term funding model they should be good. I'm still waiting and observing them.