It works for me - I interpret it as “unusually good value” - and it tends to avoid the rapid descent into rampant hyperbole that terms like “giant killer” evoke. And over 30 years in the audio hobby I’ve come across lots of products that would qualify as “punching above their weight” (i.e. unusually good value), where as literally a handful are things I consider “giant killers”.
Listed alphabetically by manufacturer (no other ordering implied), some of the more prominent ones that come to mind (and not necessarily an exhaustive list):
Audeze LCD-2.2C (circa 2012, pre-fazor)
It took even Audeze several years to comprehensively beat a good copy of these. Yes, you could argue the LCD-X beat them - at nearly double the price and with a completely different signature. But if we wanted to retain the Audeze house-sound, it wasn’t until the original LCD-4 that Audeze pulled that one off reliably (good copies of the LCD-3 could be preferable - but personally I never found one that was good enough).
Dan Clark Audio/Mr Speakers - AEON Flow 2 Closed
Probably the best closed-back planar, period, and one of the better closed-backs at any price. The only reasonable alternative I can think of to the Elegia, for those that want a meatier signature, and really not much else until you start running into the likes of the Vérité Closed and Focal Stellia.
Etymotic Research ER4-S (and SR/XR)
The best mechanical isolation available, in a linear sounding, reasonably easy to drive, and tiny package that exhibits the best phase, technical and overall tonal performance in any single-driver IEM I’ve heard, and comfortably, and realistically, competes with IEMs costing many times its asking price.
Focal Clear
>90% of the performance of the Utopia for about 40% of the price. Easy to drive. Only really serious challenger for resolution and dynamics was the HD800S until the HEDDphone dropped.
Focal Elegia
Closest thing to a truly neutral, reference-class, closed back headphone - with fewer artifacts from being closed than pretty much anything else. Even with higher-end models from elsewhere most are a “fun” (U or V shaped) tuning, or too specialized, until you run into the Vérité Closed and focal Stellia.
HEDD Audio HEDDphone
A first entrant that, after a couple of weeks listening, I would say pretty much tops everything I’ve heard around its price, several models well above its price, and manages to best some much more expensive units in some individual technical areas. Possibly redefines its segment (though may also result in price increases from other manufacturers as they update/replace their similarly-priced models).
Koss Porta Pro
Not much really needs to be said here.
RAAL-requisite SR1a
Literally bests every headphone I’ve ever heard, at any price point, all the way up to Sennheiser’s $55,000 HE-1 (“Orpheus II”). Redefined what was possible in a headphone, while coming in at a lower cost than any of the legitimate flagships.
Sennheiser HD58X - Jubilee
As a sensible remix, that are easier to drive, and have a really solid baseline tonality, along with more than decent technical performance, they handily beat out much of the competition up to 2-3x their price, with the exception of their old, more venerable, “brothers”.
Sennheiser HD650 (and HD6XX)
A classic, and its cheaper, plainer, but still fantastic, derivative. Recent models (late 2015/early 2016) removed the “veil” and expanded the stage. These are still a reference over a decade and a half (actually 17 years) since they were introduced. Even today very few headphones can legitimately claim to best the HD650 across the board, and they are frequently fixtures in systems with four and five figure sources and amps.
Sennheiser HD800S
The HD800S, along with suitably modified original HD800, have been sporting with the more recent rounds of $3,000-$4,000 headphones all along. They’re not as heavy a higher-tier puncher as the HD650, nor as consistently so highly regarded, but there are still things they do so well that you cannot best them without spending significantly more.
I’ve got a big box of “recommended” models that fall in that category that ably demonstrate that by no means does every $35-50 Chinese IEM punch above it’s weight. A lot were superficially impressive, but with further listening it rapidly becomes apparent the only punching that then went on was to the dick.
Which is not to say there aren’t IEMs in this category, its just not nearly all of them.
A disturbing proportion of Beyerdynamics, everything I’ve ever heard from Ultrasone, the HD700, Sony MDR-Z1R, Fostex TH900-Mk2, I think HiFiMAN still need to do a better job on their builds, particularly at the higher end and the Meze Empyrean would be a lot more interesting at $2,000.