In short, there isn't really a low point or a high point. The history of the papacy in terms of temporal power (which I think you're getting at) has waxed and waned quite a few times and there is no real way to tell considering the turbulent history that occurred in the Italian peninsula or Europe as a whole after the Fall of the Roman Empire.

When did the papacy reach its zenith in terms of temporal power? The answer really depends on what one means. If in terms of land mass and what was directly under control of the Papacy, then the time period of the 1700s-1800s has the Papacy dubbed the Papal States (which had much of the central region of the Italian Peninsula under the Papacy's control), but this was more of a loose confederacy of nobles who swore fealty to the papacy. But the Papal holdings and its capital Rome has been sacked by so many different factions and then restored on multiple occasions, one can't really say with confidence when the papacy was at its strongest militarily or in terms of temporal control over its holdings. (Rome has been sacked at least two times while under the papacies control; one in 1527 by a Spanish-Imperial alliance, and in 1870 to complete Italian unification).

Politically, its a bit more complicated. The politics of the time were rife with intrigue and espionage and the rulers of the time flopped so many times its hard to determine who really had the upper hand over who within a decade. The Crusades can be construed as an example of the papacy exercising political and religious influence on the foreign powers, but this is heavily reliant on whether the foreign powers were motivated to commit resources in turn for greater political power as a result of the campaigns. (If anyone would like to interject on this subject please do, this isn't really my strong suit)

Culturally is easier to define. The Renaissance Era treated the Papacy rather well in certain years. Specifically the papacy of Pope Julius II (1503-1513), which saw the commission of famous works of art by reputed artists Michelangelo, Raphael, and Bramante. Pope Leo X (1513-1521) was also considered a patron of the arts. But his selling of indulgences and the proceeds used to commission such lavish pieces of artwork led to Martin Luther writing the 95 Theses and arguably led to a significant backlash in religious influence.

The question of when the papacy reached its nadir in terms of temporal power is a bit easier to define. But they are more of serious setbacks then low points. Religion and politics have always been intertwined since the inception of the papacy (no doubt the Ancient Romans precedence of political rulers being religious ones as well set a precedence), therefore the religious influence of the papacy has a direct impact on the political influence of the papacy over other nations and peoples. The Great Schism in the 11th Century between what we now call the Latin and Greek Church created a huge setback for the papacy. This I consider a given. The Reformation in the 14th century can arguably be another political nightmare for the papacy as well. Other low points in the history of the papacy would be the Sacks of Rome by foreign powers (refer to second paragraph for some examples), the invasion by Napoleon in 1798, and the Avignon Papacy (the popes of that time were arguably under the control or influence of the French monarchy).

Note there are countless examples of lowpoints and highpoints in the history of the papacy that I can't possibly list them all. If you feel I misrepresented facts or wish to contribute, please do! This subject is actually quite interesting and I'm always learning something new in regards to the papacy!