Best: I'd have to go with my DT831's, since I got them for a ridiculous low price. Especially after sticking a new cable on them, they have become a real stand-by for me. They provide the most neutral sound of my current headphones, and serve as a nice reference point. Of course with the push Beyer pads, they are always welcoming to wear. They sound quite a bit better than any of the other sub-$500 closed headphones I've used. If I had to really downsize my rig, these are the headphones I'd keep if I could only keep one pair, because they are more practical and less susceptible to wear and tear than my A-T's, and aren't that much worse to listen to or wear.
Worst: The Ultrasone HFI-650DVD was a pretty disappointing headphone for me, though not so much due to sound. The bass was big and powerful as I expected, and the rest of the sound was serviceable. However, the small earcups and amazing heat-trapping ability of these headphones made them pretty uncomfortable. Add in the cracking headband issues, and I was less than enthralled. Fortunately I bought them used, and was able to sell them without too much loss. But I'd definitely rate it as the worst experience of any serious headphones I've owned (which is not to say it was awful, merely bottom of the barrel).
