The AKG K701: While technically a good heaphone, I must agree with Uncle Eric about their rather plasticky sound and anemic, though tight, bass. While the mids are indeed excellent, I just didn't feel that this headphone was well balanced enough, or really that great for anything other than classical music and some jazz (and even then, only when well-amped). AKG may just be my favorite company for overall comfort and performance at a reasonable price, but I just feel these headphones are really overrated.
The Equation Audio RP-22X I purchased back in my basshead days. Immediately regretted it after listening to better, more balanced heaphones. For a bass-heavy headphone, I found these to be surprisingly bright, and certainly not in a good way. Also, in spite of their rather durable plastic, the headband has a tendency to crack easily under stress. These didn't last me a year.
The Audiotechnica ATH-M50S: It might sound like I'm trolling here, but I assure you, I'm not. Sadly, I have to say that I'm not a fan of these headphones, but the reason is rather particular to me. I have a very sensitive eustachian tube in my left ear, that doesn't respond well to pressure being applied underneath and around my ear for prolonged periods of time. Unfortunately, the ATH-M50, while great for the price, clamp WAAAAY to tight for my taste, and as a result of my ear issues, I developed an ear infection that was quite painful. Hence, I have very bad associations with the headphone that I fear I will never fully overcome, although I acknowledge that they are a solid contender at their price point. Also, the highs on this headphone can be overly strident and bright for prolonged listening, and coupled with the slightly emphasized bass, cause the mids to sound more washed-out than they are in reality.
The absolute worst headphones I've ever owned were with out a doubt the first-generation of Brookstone Noise-Cancelling headphones. Horribly colored sound, somewhat tight fit, and worst of all broken noise cancellation that resulted in inexplicably loud feedback on one side of the headphone! I will never forget the terror of that high-pitched shreik creeping into the headphone when I was listening on the plane. OUCH!!!!