Node.
Node node node node node.
Why node?
Node is a great framework for building web applications. It's got good support for scalable servers using Amazon Opsworks, nodejitsu, heroku, etc.
Javascript developers are easy to find, and even with no Node experience, any developer worth their salt can get up to speed pretty quickly.
Front-end and back-end can be written in the same language.
You can build rapidly and get to an MVP very quickly, at which point you can either switch to an enterprise language like Java (ugh) or Go, or whatever else to leverage performance, or scale your existing Node codebase using modules such as cluster, or by developing simple strategies on multiple servers.
I have built every kind of applicaton you can think of using Node, from desktop apps using node-webkit, to simple TCP video streaming servers using the built in networking and piping data directly into ffmpeg.
Testing and test driven development is natural in node, and you can write your server tests and browser tests in the same framework using mocha and phantomjs. I've even been building integration tests and automated UI tests using node, mocha and pahntomjs.
Our latest concurrent benchmarks for a load balanced group of t1 micros on a single mongodb instance get to at least 30k concurrent requests a second before you get a few timeouts, but that's just because i'm not too familiar with ops and haven't done any proper optimisation yet.
There is no right answer to this quest, but the above is my personal and professional opinion.