the highest point of something
pinnacle - peak
The word ‘pinnacle’ was originally used to refer to something physical, such as the peak of a mountain, or some other formation or structure. It has since been adapted to also express the most successful, admired, or exciting part of something. It is in this latter context that it is most commonly encountered.
I look back on that achievement as the pinnacle of my career. It was the culmination of years of hard work.
After winning the election to become Prime Minister, the politician achieved his goal of reaching the pinnacle of British politics.
After an arduous twenty thousand foot climb, we reached the snowy pinnacle of the mountain.