Douglas Harper's Etymology Dictionary
Wiktionary
a. Not surpassable; unable to be surpassed.
WordNet
adj. not to be exceeded; "unsurpassable skill"; "unsurpassable standards of workmanship"
Usage examples of "unsurpassable".
Luscious, they promised to fill the mouth with a sweetness, a flavor, a succulence unsurpassable.
God was the fire of love kindled in the burning soul that, thusly transpierced with the dart of the assailing Seraphim, thereupon felt the wound with unsurpassable delight, was made radiant by it, was allowed by God at times to manifest that divine sore outwardly.
Some for the Bunions that afflict us prate Of Plasters unsurpassable, and hate To Cut a corn--ah cut, and let the Plaster go, Nor murmur if the Solace come too late.
Each individually selected birch twig in the broomtail has been honed to aerodynamic perfection, giving the Firebolt unsurpassable balance and pinpoint precision.