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)