Which part has the Best Prime Rib Roast
Actually, neither. The part you want is the middle. The best single piece of meat on the cow is actually the "cap" of the rib roast, technically the spinalis dorsi muscle. A rib roast runs from ribs 6 to 12, but the spinalis dorsi is thickest in the middle, say from about ribs 8 to 10.
But if you're only intersted in comparing front or back, also, respectively, large or small end, the back (loin, or small) end is considered more tender, the front more flavorful.