The cheapest and lightest one you can find (i.e thinnest). Light so it is easy to use and cheap because in my experience, the thickness and cost has little to do with cover life. I have had a 12 mil, 8 mil and now a 5 mil and they all seem to last about two seasons for me. But the 5 mil is so light anyone can remove it and put it back on so it is more likely to be used.