Well it depends a lot on the place you look at. England had their early parlamentary system that gave the nobles a lot of power but the Kings didn't lack power at all.

In France in the late middle ages and early modern time the King completely stripped the nobles of any significant power outside of their small lands

In the HRE the Emperor was quite powerless in comparison because he was oberlooked by a group of high worldly Lords and bishops. Also he heavily relied on the pope's support.

In the 15th C the rise of the Medici family in the Rep. of Florence can probably be considered to be an example of how powerful the noble classes could become. Banking family turned royal house, the Medici's had 3 members become Pope and we're essentially sovereign rulers of Florence, arguably the most important part of Italy during the period.