Worst headphones hmm got a fairly big list and only restricts to full-sized can's.
Sennheiser
HD515, 545, 570 - schitt treble very recessed, 570 - brighter than your usual Grado SR-80
Koss
UR50, DJ100, all Pro4aa/aaa's ( 60-70's vintage, where fak my mids, highs go? And modern models due to schiit earpads) Koss ESP6 and 7 (I don't care how you restore them or mod them, crap is going to be crap),
Sidenote: Not really crap but out of the stock crappy energizer the ESP-950 deserves a more better energizer than the stock.
AKG
K300, K/Q70X (I don't care if you're powering these out of a $5k amp, you can't fix that crappy plasticky sounding treble, soundstaging and imaging is alright, good looks and best to get rid of the stock drivers ASAP)
Grado
SR-60, 80, 125 (SR-60/80...my ear's...my faking ear's..to bright can be fixed by full driver housing modification Billavideo started a thread on few years ago, I'm very surprised that Alessandro's don't have this bright glare problem the SR-60/80's have made by the same company). SR-115, What happened to my soundstage? Also bright sounding, skip this model and get a 225 or 325i.
Denon
D2k, D7k and all the new Denon range. (Few years back a few user's called the D7k natural sounding...What?? Like Duggeh once said, these are bass-bloated fart monsters....extremely coloured warm and bassy, but very nice build quality thanks to the headband and wood, D2k - if you want bass, quality bass look elsewhere and skip out on the D2k's. Don't care if it's markl modded or got plutonium rings modded to the driver, crap sub-bass and mid bass performance, my HD438's modded can out sound perform them). Surprisingly the D5k (after screen mod) is the good sounding Denon out of the range.
- Since the D2 - D7k have been discontinued they were replaced by the new D***/k line...these are ugly and sound just as bad as the original predecessor models....Denon need's a visit to the doctors.
Superlux
That one 668 model....sounds too artificial, can be fixed with some equalisation....how can people call these neutral and better sounding than the original K240 models?
Monster
All the Beat's line except the Studio and Pro's. For the price they are very bad but the higher-end models namely Studios and Pro's especially don't look that bad, the Pro's are built fairly well, nothing to write home about though. The pricing on anything by Monster is an insult us audiophile, hipsters, head-fi, me myself and I and sheep followers of the masses after fashion.
Ultrasone
Niche products for niche people, at least in my opinion.
Pro900, hyped and overrated acclaimed "godly bass performers", sorry but I once heard a slightly modded M-Audio Q40 to out sound the Pro900's. Very dark and bassy, you pair it with a warm sounding amplifier and you don't have sound left besides distortion.
Limited Edition 8, 10: Pricing on these are absolutely ridiculous for the sound performance, my overrated LCD2's sound better and is still cheaper. Although besides sound very good packaging and build quality though, absolutely love the leather earpads on the Ltd Ed 10's and the included headphone stand, would definitely buy one for looks only.
Audio Technica
W5000, hear one of these and quickly learn about sound not living up to the promise of the price.
M50's, probably the most overrated headphone on head-fi since Nikola Tesla invented the formula for electricity.
That's about it for headphones. These are all compared and devalued by me sound-wise....build quality wise and IEM's is another story for another thread that I don't want to get started on at all.