[sandreas]

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)