Not able to be compared.
So much better than another as to be beyond comparison; matchless or unsurpassed.
Something beyond compare; a thing with which there is no comparison.
That cannot be graded.
(of an adjective) Describing a quality that cannot normally be varied because it is extreme (e.g. freezing), absolute (e.g. dead), or classification (e.g. nuclear).
Any adjective that is not gradable.