UPDATED: After achieving a proper seal I've been extremely satisfied with the K550.
There's been more, but the following are the headphones that I feel I've had in-house for long enough and spent enough head time with to draw a conclusive assessment of their overall performance:
1. Sennheiser HD600 w/Cardas cable -- There's simply nothing "wrong" that they do to my ears.
2. Ultrasone Edition 8 -- Stunning from top to bottom with the right chain of components and after accruing some hours on the drivers. Middling without the prerequisites met.
3. Stock Sennheiser HD600 -- Still great with the stock cable and even a touch more airy than the Cardas, but tonal weight across the board is noticeably lacking in comparison.
4. AKG K550 -- Addictive clarity combined with a great sense of space for a closed headphone. A bit thin sounding unamped, but when paired with complimentary components that sufficiently stimulate the drivers they're analytically-inclined goodness. Absolutely requires a good seal to perform to their potential.
5. Grado HF-2 -- Beautifully done bass perfomance. Not the most "accurate" sound, but thoroughly engaging and enjoyable.
6. Terminator V4 -- A variation on the Darth concept, the midrange is moved forward (still a touch recessed, however) and the bass is reduced a few decibels. It's still incredibly fun, however, and is the closest I've heard to a headphone sounding like a room of loudspeakers.
7. Darth Beyer V3 -- Possibly the most "fun" headphone on this list, it's the undisputed king of bass. Beautiful tonal enhancement with some records and a horrible match with others.
8. Ultrasone HFI-580 -- Vastly underrated and often mis-labeled as a purely "basshead can" in my opinion, it has a very clean, detail-oriented sound that's a touch bright of neutral. Prone to over-amplification but opens up nicely with the right amount.
9. AIAIAI TMA-1 -- A functional fashion statement, they're definitely tuned for DJs with their forward mids and treble. Add some EQ to the high frequencies and they come to life nicely. A smooth sound from top to bottom that's still nicely detailed.
10. Koss KSC75 -- Possibly the best "bang for the buck" on this list, they can be found for around $15! Lacking in extension on both ends, but overall a nice tonal balance and acceptable clarity. I've always liked clip-on headphones for the gym and these are by far the best I've owned. (although, I refuse to invest in something that'll definitely be getting banged around) A perfect headphone for those of us who want good portable sound but see portable hi-fi as having a poor return on investment.
11. AKG K271s -- The can that brought me to Head-Fi, they do well on the details and never sound like they "enhance" a particular frequency. Bass presence is lacking and they can sound sterile, however.
12. Ultimate Ears TripleFi 10 -- These could possibly be a few spots higher on my list, but I'm simply not a fan of IEMs due to comfort/fit. Nice clarity overall but soundstaging was the smallest on this list and treble was too emphasized to my ears.
13. Audio Technica ATH-M50 -- A bit of excitement in the bass, (NOT basshead material though, in my opinion) slightly recessed mids, and some grain in the high end. Otherwise a great value.
14. Grado RS-1 -- No disrespect to these much-lauded headphones, but they certainly weren't for me. A small, congested sound with biting highs. I went in optimistically expecting a more refined HF-2, but I just couldn't find much of anything to like beyond the aesthetics in all honesty.
15. ALO-Modded Ultrasone HFI-780 (ALO-780J) -- Waaaaaay too forward and metallic treble for my liking, reaching a sufficient listening level was a chore. I've never heard these stock, but if they're half as aggressive as these they'd likely end up in the same position.
16. Ultrasone PRO-900 -- More comfortable than the HFI series offerings, but with serrated treble akin to the ALO-780J and some of the most problematic bass I've heard on a headphone. Simplistic, non-trebly electronic music sounds fine, but deviating from that niche caused these to transform into an amalgamation of highly unnatural/painful treble, uncontrolled bass, thin mids and excess grain.