Form-fitting is best, given that a tight shirt is less likely to catch on a branch or nail or whatever. Long sleeves will protect your skin from minor scrapes and, if required, the sleeves can be torn off and used as a bandage or tourniquet.
As for bottoms, cargo pants are the perfect article of clothing. The extra pockets can be used for storing equipment and cargo pants are typically cut loose enough to run in but not so baggy that they might catch on something.
A few Slasher films in which the above advice was not followed: