I have used kite tyvek (heavy, too much volume in backpack for me), 4 mil painter's plastic (not durable, high volume in backpack for me) and polycryo (tough, stuffs into small stuff sack).
I have also purchased window film: it isn't even slightly like polycryo.
I will try other products because I don't like the price of the polycryo.
I keep going back to the polycryo because it is tough and it will stuff in a small stuff sack.