High value, giant killer speakers?

Magnepan LRS without a doubt.

Incredible sound for very little money.

They are not for everyone for sure but, certainly fit the criteria for this post. 

Doug 

As others have already said, Tekton.

I have heard several, and they outperform speakers at much higher prices. 

Also, Maggies. But they have more limitations in set up, and driving them.

Their top of the line, 30.7 at 30K (hardly cheap) still outperform speakers at 2X or maybe more. 

The Wharfedale Evo 4.2.....i've never heard a speaker for under $1500 that can do what they can with the music, I know the usual suspects like the Maggie LRS, Zu Dirty Weekends, get a bunch a praise. These are a very serious speaker multi-layered cabinet beautiful veneer, Larger than usual AMT tweeter, soft dome mid-range, and weigh in at 30 pounds a piece a pretty big Standmounter....

Any speakers that are the entry-level of any manufacturer tend to offer much more value than their more expensive models. Pretty much like cars. A 3-series BMW or a Cayman offer a huge slice of the performance of their costlier brethren. 
I would say "high-value giant-killer speakers" are just that.
Or buy used, always buy used to get the most bang/buck.

I'll pile on the Maggie train here.  Used Thiels can be awesome and can be had for 25% of their original price and there's a whole cult of Thiel users out there.  I would also say that your choice may also depend on the type of music you listen to.  Maggie and Thiels both are capable of great delicacy and are highly resolving speakers -- great classical and jazz speakers.  Don't let anyone tell you Maggie's can't generate high SPLs because they very much CAN and will but require as has already been stated, large amounts of current to deliver.