What cable and charger do I need to get the fastest charge speed on my Samsung Galaxy S8?
Which Galaxy S8 do you have? 

The version with a Qualcomm Snapdragon supports QuickCharge 4, which is backwards compatible. 

For the cable choice, QuickCharge 3 used 2.5A on 5V to charge. So your cable and charged need to support that amperage. What QuickCharge 4 uses is unknown so far.

In order to achieve the maximum you need usb 3 with a type C connector (specified for 3A). However many manufacturers sell usb 2 charger with 2A or even more, including Samsung with their fastcharge. The cable itself does not really matter, however, to be save buy a 3 or 3.1 cable, since those were built for 3A of power.