One that fits! If you're purchasing screwdrivers, just pick up a screwdriver set (they aren't expensive for a typical set, £5-£10) and use whichever screwdriver works with the least amount of pressure required. Use a Philips head, posi-drive tend to round off the the screw heads.
Most of the time it's a 0 but some heatsinks use a #1 for the 4 screws that hold the heat spreader to the GPU socket.
https://www.amazon.com/Stanley-66-052-6-Piece-Precision-Screwdriver/dp/B00009OYGV Best little precision set out there, used mine at Asus.