WWII Russian snipers? I don't know. There are many different ways to go about who was the "best", but really there is no way to compare Hannibal, Alexander the Great, Saladin, Genghis Khan, and so on to every era of soldier and war since. You can only weigh them on the merits of the way they fought in their conflicts. So best of all time? Whomever is the best at this current time. No past army would defeat them.