This thread is exactly the same reason why I joined the Head-Fi forums because I, too have had my share of bad experiences while looking for headphones. However, the winner for the worst I've heard was a pair of COBY headphones I bought to replace a pair of Lenoxx Sound headphones I could no longer re-solder in 2005. The COBY headphones, aside from being uncomfortable were muddy and murky. Highs and mids were damn near non-existent, and they were overly bassy, and this was before getting ten pairs from Lenoxx Sound that same year. Sadly, I'm down to my last two pairs from the ten that I got, with my best pair in my best friend's house, (as I left them in his car, Friday night) and my secondary pair, which needs to be resoldered, leaving me to rely on a pair I bought from Encore Data Products, last August.
While these aren't the absolute worst, they're not the greatest I've heard either. While the bass is decent on these, they're somewhat muddy, and have no highs or treble either. My biggest gripe is that they're what I consider "deaf headphones." --By "deaf headphones," I mean a pair of headphones that have a really soft output on the highest volume setting of my laptop or HTC Glacier (MyTouch 4G smartphone) This is most notable when I'm editing audio in Adobe Audition, or listening to .mp3's saved in my HTC Glacier, on the bus ride to and from school/work/etc.
I honestly can't wait to get my best pairs back, along with a dozen Lenoxx Sound headphones I bought from Cyberguys, yesterday.