Oak is considered the best, but maple, beech, and a few others are supposed to be almost as good. I always just throw a mix of hardwood leaves in my enclosures and the roaches can pick out what they like. Oak always seems to disappear the fastest though.
I don't know where this whole "Oak is the best" idea came from. There is only one oak tree within 50 miles of me, and I tried using the leaves from it in my roach colonies. The roaches preferred the ash leaves from my yard. It doesn't make that much of a difference IME though.