The thinner the better, but it depends on the actual gap between the chip and the heat sink when it's installed.
If you didn't take the old pads off, you can go off that. Otherwise, order several thicknesses and start from the thinnest, then see what impression it makes on the pad after installing the heat sink. If there's no impression or it doesn't appeared to have squished, the pad is too thin.
Should be good to start with0.5, 1.0, 1.5 would be good to buy?