ASP.NET Core, it's a great ecosystem with great tooling, and C# is fantastic language to work with. I develop on Windows, deploy to Linux with Docker, docs are great too.

This strongly depends on the purpose...

  How is it sized?
  Can it get bigger?
  Is it realtime?
  Is it Web / App / Other frontend?
  What backend languages / tech is supported?

I always wanted to try or tried successfully:

  SignalR (C#)
  Flask (Python)
  Gorilla (Golang)
  Revel (Golang)
  API Platform (PHP)
  Hapi (js)
  Express (js)
  Actix-web (rust)

I've been really happy with express (Node). It has scaled really well for my company.

Currently processing almost 200 million requests a month on a cheap $25/mo heroku dyno.

I've been wanting to try nhost, but if it's something you'd want to put in prod fairly quickly, I'd go with Firebase.

I want to try postgrest.

Rails.