Overall? Theoretically it's probably the Hitman's Heatmaker, being the best sniper rifle in the hands of a sniping god (Albeit, probably less reliable in the hands of someone without thousands of hours on sniper).
Disregarding sniper however, my vote would probably be the Stock Scattergun. It's reliable for any level of play, great against any class in 1v1 combat, and benefits Scout's strengths immensely. It helps Scout dominate against any other class in close-medium range combat in the hands of a decent player. The Soda Popper might be the better overall primary in the hands of a good player, but the reliability and power of stock allows it to remain useful regardless of your level of play.