DB: Postgres

API: Ruby on Rails

Frontend: React, the react_on_rails gem to set up webpack, Flow, Redux, Immutable.js

Background Jobs: Sidekiq and Redis

Caching: Memcached

PaaS: convox (on AWS). See: https://convox.com and https://github.com/convox/rack (But you should start with Heroku.)

I recommend the Rails stack. It's not the "hot new thing", but it's very stable, secure, and productive.