What term describes something which is ubiquitous and consequently poor in quality, but occasionally exceptional and noteworthy?

In order to capture the juxtaposition of the truly sublime and the mundane it rises above, perhaps an idiomatic (or at least semi-idiomatic) phrase like a diamond among the pebbles or a diamond in rough pebbles, or whichever variation of it seems more idiomatic to you, could work.

I think I understand what you are after. You describe a scenario in which a person bites into THE cookie, not expecting anything other than the usual mediocrity. S/he is pleasantly surprised, however, to discover it is the quintessence of chocolate chip cookieness.

I would call that moment an epiphany, an eye-opening experience. It is an experience that breaks the mold. It serves as a sort of paradigm shift in the way the chocolate cookie is henceforward perceived. That cookie becomes a benchmark for tasting and judging any other cookie, yea, any dessert. It is the lodestar, the zenith, the epitome, a game changer, da bomb.

Is this what you are after?

I'd propose "Exceptional" in this scenario.

unusually good; outstanding

I think it works very well to convey the fact of being better than usual... i.e., that the quality is an exception to the usual standard.

The team played well but Jones' performance was exceptional.

This clearly shows one to be a "stand out" from the norm.

You do say "one isolated instance" in your question so you could go with "unprecedented".

There are several words to describe something which is far above average, or normal, quality.

• Superior
• Excellent
• Outstanding
• Best
• Premium
• First-class

In my opinion these words get overused, and so their meaning is sometimes diluted by silly advertising. If you want a word that expresses that the cookie exemplifies the best possible, you might use:

• Superlative (though it might sound a bit funny)
• Supreme
• Divine
• Peerless
• Unparalleled

"This cookie is outstanding! I worry I will never taste one so delicious again."

"This is the best cookie I've ever tasted."

"This cookie is divine."