Lots of definitions for giant killers, I suppose, but here’s my experience and what shook out as giant killers for me and the reasons I’m willing to suggest them as such.
I went thru a 6 month speaker search where I auditioned a large number of speakers, probably over 30 but for sure over 20 (didn’t keep detailed notes).
In the end for me Spendor D7 were my giant killers because they, for me-let me stress that, bettered a long list of comparatively priced speakers (giant here in beating quantity) and bettered for me a small group costing well more (804d3, persona, kaanta), (giant here in beating well known popular speakers costing nearly double).
Your results may vary but that was my experience. I’ve written extensively on the search in other threads so if anyone wants more detail just see my past posts.