Gai. Eight Gates is too much for Darui to overcome, especially since he's not really a close quarters type. He would get his ass killed, and Gai would only need to open up to the Third Gate at most.
yeah like i had no idea how strong guy really was untill he took down kisame but now he rules like the only downfall about his 8 gates is after the fight he is almost dead lol but tbh like anyone will survive guy with the 8 gates ? tbh he even started giving it a good go against tobi so i think he is pretty awesome the only other thing you would have to worry about is darui's black lighting style .. its pretty sick ! haha