Interestingly, in Washington's time, there were no 4, 5, or 6 star generals. So when the General of the Armies rank was created, Washington was given a posthumous promotion, so he would eternally have the highest possible rank. The only other man to hold that rank is Jack Pershing for World War I.
This quiz has already been spotlighted. I guess that's the best we can expect for a quiz like that.