Here's the three answers you will get when try looking deeply for it:<p>1) Quick prototyping old-school: Ruby with Rails or Python with Django<p>2) Asynchronous Single Page App: Node.js using either something like Meteor (fullstack) or combining something like Express(backend) with Angular/Ember<p>3) You already have big clients, you need 'scaling' immediately: Java or Scala with one of their frameworks.
Assuming there is a next startup and it's anything like the current startup, then I'd probably go with Groovy again. Groovy has worked very well for us.