the best stage of development — as in "I met her at the pinnacle of her success."
a high pointed rock formation or mountain peak — as in "The eagle circled the rocky pinnacle."
a pointed tower at the top of a building — as in "With the pinnacle, it is the highest building in the city."
They congregated round me; the unstained snowy mountain-top, the glittering pinnacle, the pine woods, and ragged bare ravine, the eagle, soaring amidst the clouds—they all gathered round me and bade me be at peace.
Chapter 10 (10% in)
There are no more uses of "pinnacle" in Frankenstein.