All of them! 
Depends on exactly what I want the camera to do for me. I'll happily shoot M and if I'm chasing dogs or kids around then sometimes I'll use S. If I'm shooting landscapes I'll use A to control just the DOF since I'm usually using a tripod and shutter speed does not really matter and I'll even use P for some quick and dirty flash work.
If you stick the camera on P and dial in -2 exposure comp and fire that flash it's the quickest and easiest off camera flash you will ever find for those dramatic outdoor saturated background style shots
So all of them.
Depends on exactly what I want the camera to do for me. I'll happily shoot M and if I'm chasing dogs or kids around then sometimes I'll use S. If I'm shooting landscapes I'll use A to control just the DOF since I'm usually using a tripod and shutter speed does not really matter and I'll even use P for some quick and dirty flash work.
If you stick the camera on P and dial in -2 exposure comp and fire that flash it's the quickest and easiest off camera flash you will ever find for those dramatic outdoor saturated background style shots
So all of them.
