The Collaborative International Dictionary of English v.0.48:

Stunner \Stun"ner\, n.
   1. One who, or that which, stuns.
      [1913 Webster]

   2. Something striking or amazing in quality; something of
      extraordinary excellence. [Slang] --Thackeray.
      [1913 Webster]

WordNet (r) 3.0 (2006):

stunner
    n 1: a very attractive or seductive looking woman [syn:
         smasher, stunner, knockout, beauty, ravisher,
         sweetheart, peach, lulu, looker, mantrap, dish]
    2: an unexpected and amazing event; "the stunner was what
       happened on Saturday"

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "stunner":
   Miss America, amazement, astonishing thing, astonishment,
   bathing beauty, beau ideal, beaut, beauty, beauty contest winner,
   beauty queen, belle, bunny, charmer, cover girl, curiosity,
   dazzler, dream, enchantress, exception, gazingstock, great beauty,
   knockout, lady fair, looker, marvel, marvelment, miracle, model,
   nonesuch, paragon, peach, phenomenon, pinup, pinup girl, prodigy,
   pussycat, quite a thing, rarity, raving beauty, reigning beauty,
   sensation, sex kitten, sight, slick chick, something else,
   spectacle, wonder, wonderful thing, wonderment