pinnacle
Meaning: the highest point
Usage: Book reviewers declared that the author’s new novel was extraordinary and probably the pinnacle of Western literature.
zenith
Meaning: the highest point, culminating point
Usage: I was too nice to tell Nelly that she had reached the absolute zenith of her career with that one hit of hers.
nadir
Meaning: the lowest point of something
Usage: My day was boring, but the nadir came when I accidentally spilled a bowl of spaghetti on my head.