First I think joysticks would work the best with buttons for shooting gernades and crouching and the ability to customize the sensitivity and placement of buttons and joysticks the problem with fps games on iPhone is that most of the time I want to aim shoot and move all at the same time and without bumper buttons that most controllers have it is pretty hard
I think the iFPS controls but with the entire side of the screen for each joystick would work very good
