Given that the Middle Ages lasted about 1,000 years, it is hard to say that there was one major political value in all of Europe for all of that time. However, if I had to pick one value and say it was the major value, I would pick feudal loyalty.
Feudalism was the dominant political form in Europe during most of the Middle Ages. The main value that underpinned feudalism was loyalty. Loyalty in the feudal system was supposed to work both upward and downward. Vassals were supposed to be loyal to their lords. Lords were supposed to, in a way, be loyal to their vassals by protecting them and providing justice for them.
This value of feudal loyalty was often ignored, but it was, ideally at least, the dominant political value in Europe during this time.