WordNet sense 1 (the single one of its kind):

singular

WordNet sense 2 (existing alone or consisting of one entity or part or aspect or individual):

single

WordNet sense 3 (radically distinctive and without equal):

alone, unequaled, unequalled, unparalleled

WordNet sense 4 (such that comparison is impossible; unsuitable for comparison or lacking features that can be compared):

incomparable, uncomparable