The right cross (for an orthodox stance, left for a southpaw) or right overhand actually generate the most power, and it's not even close. If you ever see fighters have their power measured, the cross will generate more than double the power of a hook and triple the power of an uppercut.
The uppercut and the hook generate significantly less power, but the uppercut usually lands right on the chin, which is the perfect spot for a knockout, while the cross is more likely to make contact in a glancing fashion. Hooks, my personal favourite punch, are excellent because they surprise opponents and land either on the jaw, chin, or temple if they are aimed at the head.
Nonetheless, if you are trying to generate power, it is the straight punch travelling the greatest distance and allows you to get all your weight into it that wins by a long shot.