Brainleisttt ! -did any one person or class hold the most power during the middle ages, or did it shift back and forth?

I would disagree. The answer would be the upper classes, lords, kings, that sort of thing. They held dominion over the other two, and so held the most power. They owned the most land, more than any small farmer, and employed the middle and lower classes to work on the farms to sell the produce, take part of the profit, and provide for the rest.