Humdinger

Noun

 * 1) Something that is particularly outstanding, unusual, or exceptional.
 * Most of the questions were pretty easy, but that last one was a humdinger.

Thesaurus
ace, beaut, corker, crackerjack, daisy, dandy, darb, dilly, dream, honey, killer-diller, knockout, lollapaloosa, lulu, nifty, peach, pip, pippin, sweetheart, the nuts, whiz

Etymology
Of obscure origin.