Every situation will be different, but you cannot really go wrong with low range 2nd or 3rd gear and diff locked.
Top tip, if you damage their cars every time you recover them, then they stop asking!
Whatever you do, make sure their wheels are being driven when pulling out of deep mud. I accidentally disengaged my central diff on my TD5 90 and although I gave the engine revs the drive shafts weren´t rotating. As I winched it out the hand brake drum dragged along the mud and bent the output shaft on my transfer case.
Or better still pull it out in the same direction that it went in