i ski keystone almost every day, so i'll speak up..
keystone is usually the least crowded, specifically the park, early in the week. m-w its fairly empty, thursday and friday it picks up a little more, and the weekends are the worse, but still never bad compared to other places. (see: breck) the cool thing about keystone is that even when the main parking lot is completely full to the point of being closed, when you get out on the hill (and even at the lifts) it never seems that crowded. skier's left is closed for the season because they're rebuilding for superpark, so you're left with skier's right which is all of the smaller jibs and medium jumps, but its still fun. hope i helped.