The best front fenders are those that fit in the bottom of the steerer tube of the fork (others don't give you enough clearance off-road). I still found that the guard (tried a couple of brands)was interfereing with the brakes on braking/fork compression and am currently running a down-tube guard (crud-catcher) that keeps more spray off me than I expected. Rear wise look for a guard that's fairly long and reasonably wide, whilst giving at least an inch clearance from your largest tyre (full guards aren't any good off-road but work well on-road - I like the Crud rear guard (seatpost mount)).
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Currently riding an MTB with a split personality - commuting, touring, riding for the sake of riding, on or off road :)