Hamstrings are usually the most important. Do them the night before, and maybe some leg swings on the mountain. (Im naturally not flexy, so stretch like crazy).
A lot of people also do quad stretches. Another one to focus on is your Hip Flexor muscle. Lunges help heaps to keep these loose. Pulling a Hip Flexor sucks balls.
Its not all about stretching though man. Warming up is just as important, so is regular exercise.
lol im confused too
I ride my road bike to the hill when the snow's off the road. Feels so good to be fully warmed up before shredding.