What is ...
one of the most famous (IMO) verses from the Chandi ?
the verse that many people have heard and chanted without knowing
that it is in the Chandi?

It is one of the most beloved verses that praises the Goddess and is
taken from the eleventh chapter of the Chandi.

And it is

"sarva mangala mangalye shive sarvaartha saadhike
sharanye tryambake gauri naaraayani namostute"

Swamiji has translated the meaning as

sarva mangala mangalye - To auspiciousness of all auspiciousness
shiva -to the Good
sarvarrtha saadhike - to the [bad word] of all objectives
sharanye - to the Source of Refuge
tryambake - to the mother of the three worlds.
Gauri - to the Goddess who is Rays of Light
Naaraayani - Exposer of Consciousness
Namostute- We bow to you.