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.
It's the workhorse of the internet. It's also easy to source developers from just about anywhere.
react or mithriljs
Obj-C on iOS. Java on Android. Native looks, native performance.