While there is no sure way of knowing who the best Auror is, because of how vague the criteria to decide this is, there are several contenders in the Harry Potter book series that would befit this status. One of the major names would be Alastor Moody, who was a war hero and a veteran Auror known to put a lot of Death Eaters in Azkaban. He was fierce and very powerful. Then, there's also Kingsley Shacklebolt, who was also very strong and went on to become the Minister of Magic. Even Nymphadora Tonks was a very powerful and skilled Auror who just after giving birth to her son, came to fight in the Battle of Hogwarts.