Mudguards on a Mountain Bike - whatever next
Seriously though I have tried a variety of styles and the only one that was effective was the Mudhugger but unless you are extremely lucky the fit isn't ideal and the loading on the mounting point is pretty high if your using your MTB for its intended purpose - mine chewed through the protective film in no time and as soon as it does it moves around and the chewing accelerates, fortunately I caught it before it started on the frame.
I found the ideal solution is a Dirtlej Dirtsuit Onsie - might as well look like a 5 year old if I am behaving like one!