humdinger
Something that is particularly outstanding, unusual, or exceptional.
informal
Most of the questions were pretty easy, but that last one was a humdinger.
Where it comes from
US origin, perhaps a blend of hummer (“something that moves fast”) + dinger (“something outstanding”). First attested in a newspaper article in the Daily Enterprise of June 4, 1883, at Livingston, Montana.
Adapted from the Wiktionary entry humdinger (CC BY-SA 4.0).