The Collaborative International Dictionary of English v.0.48:

Nonesuch \None"such`\, n.
   A person or thing of a sort that there is no other such;
   something extraordinary; a thing that has not its equal. It
   is given as a name to various objects, as to a choice variety
   of apple, a species of medic (Medicago lupulina), a variety
   of pottery clay, etc.
   [1913 Webster] Nonet

WordNet (r) 3.0 (2006):

nonesuch
    n 1: model of excellence or perfection of a kind; one having no
         equal [syn: ideal, paragon, nonpareil, saint,
         apotheosis, nonesuch, nonsuch]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "nonesuch":
   abnormality, amazement, anomaly, astonishing thing, astonishment,
   champion, choice, chosen, conversation piece, cream, curio,
   curiosity, elect, elite, exception, fat, flower, gazingstock,
   ideal, improbability, jewel, marvel, marvelment, miracle,
   museum piece, nonpareil, oddity, optimum, paragon, phenomenon,
   phoenix, pick, prime, prize, prodigiosity, prodigy, queen,
   quintessence, quite a thing, rarity, select, sensation, sight,
   something else, spectacle, strange thing, stunner, superlative,
   the best, the best ever, the tops, the very best, wonder,
   wonderful thing, wonderment