Quadlock is the best solution for iPhones. They also have cases for some Samsung devices but there is always a universal adaptor...
I currently use the out front mount (it replaced the basic bike mount almost from the day it was available) with the iPhone 6/6s plus case (last year I used a 6 plus iPhone and this year the 6s plus - same case). I have installed the out front mount backwards so that the device is just above the stem...