Guys, what's the most popular cross-platform mobile app framework out there?

We recently ported our apps from Flash to Xamarin.Form..

React native seems really nice, haven't used it though

React native if you want almost native efficiency, but I really advise you to give ionic a try. It comes with all the goods of angular and you can style the app anything you want. React has very limited styling options. Anyway, the best way is to pick one that fits your project best.

I've been liking the Intel XDK, Cordova, jquery mobile combination myself.