BackEnd
laravel
Modern php framework for webapps. It's got most things a webapp will need out of the box which makes getting the service up and running that much faster.
Got a favorite technology you want to build your backend in? Need the service written in rails, nodejs, or clojure? Tell us your needs so we can make things work for you.
php
It's the workhorse of the internet. It's also easy to source developers from just about anywhere.
FrontEnd
react or mithriljs
Both libraries uses plain old javascript for templating. Not only does this make integrating with other javascript libraries straightforward, it also allows page rendering on the server. This is a must for SEO.
MobileFrontEnd
Native code
Obj-C on iOS. Java on Android. Native looks, native performance.