If you’ve ever seen a Looney Tunes cartoon, you’ve probably seen the fictional Acme Corporation’s products. Acme makes “Earthquake Pills,” “Dehydrated Boulders,” and the “Iron Carrot (Fool Your Friends!)” Many real companies are also named Acme — if you can still find a Yellow Pages, open it to the locksmiths or contractors, and you’re likely to find plenty of local businesses using the name. It’s a good one, because it comes very near the beginning of the alphabet, and it means “highest point, summit, peak.”
Zenith was a popular television brand in the 1980s, and still sells consumer electronics today. Here’s an old-school ad from the company that developed the modern remote control:
A zenith is a high point or culmination, or, literally, “the point on the celestial sphere vertically above a given position or observer.”
In sum: an apex is usually the top of something like a mountain, and a zenith is a high point in the sky, but they can both be used metaphorically to refer to the peak or top of anything. For instance, He’s at the acme of his career, or Our relationship reached its zenith during our Hawaiian vacation; it was all downhill from there.