We use the Corona SDK. I've used it personally since 2010 and then when I formed Glitch in 2012 it made sense to continue using it. It's strictly 2D ( well there's some perspective stuff you can mess with to get 2.5d sorta ) and uses Lua. It's lovely to work in ( both for prototypes and full games ) and has a fantastic community. - https://coronalabs.com/