Been far too long since I added to this, and a recent headphone has really turned the tables.
Top five: All of these see regular usage and eartime.
1: NAD RP18 Mylar (Fostex T50v1 OEM): Absolutely incredible, these are so far ahead of my other headphones that it's not even funny. Lightning fast attack and excellent decay. Totally flat response from 20-1000Hz with a bit of dropoff in the treble. Wonderful treble and bass extension, and extremely clean sound throughout the whole spectrum. Astonishing detail retrieval and control, and extremely precise imaging. Sounds like a wall of sound with the stock pads, however. Somewhat heavy, very thin supra-aural earpads, and a high level of clamping force. As expected, not very comfortable. Difficult to drive, but not restrictively so. Responds well to pad-rolling. Very pretty design.
2: Audio Technica ATH-8 (Signet TK33): Very nice-sounding cans. Extremely flat response with excellent treble extension. Warm, but a bit bright. Electrostatic attack speed and imaging, and likewise very low distortion. Nice little intimate soundstage. Featherlight design, suspension headband, and very soft velour earpads make these the most comfortable cans I've ever worn. This also makes them a bit flimsy build-wise, but still better than most modern cans. A bit lacking in the bass department since they're stats. Somewhat sloppy decay, oddly enough, and a bit lacking in micro-detail. Very difficult to power, requires (good) speaker taps to get good volume levels without clipping. They are currently being woefully underpowered, so take this evaluation of them with a grain of salt.
3: KOSS Pro/4AAA (heavily modded): Great all-rounders. Very smooth sound with downright excellent bass. Very bassy, but never dark or veiled. Does extremely well on bass-heavy music but still manages to make other genres very listenable. What little it lacks in upper treble extension it makes up for with speed and impact. Great soundstage, good imaging. Gets very slightly congested on complex passages, but no more than an HD600 does. Huge and heavy, somewhat unweildly, kind of uncomfortable. Somewhat difficult to drive. Absolute ******* tanks, these could get run over by a car and survive.
4: Pioneer SE-500 (modded): Remarkably stat-like. Sparkly, extended treble without splash, grain, or distortion. Absolute smoothest sounding headphone I've ever heard. Very lush and "wet" sounding, as opposed to the somewhat dry sound of the TK33. Chameleon soundstage with good (if somewhat diffuse) imaging. Stupidly difficult to drive and no bass to speak of whatsoever, yet somehow very listenable. Comfort is decent, build is good.
5: Sony DR-Z5: Downright excellent portables. Good attack speed and excellent decay. Ruthlessly analytical and revealing. Very flat sound, good bass extension, decent slam. Excellent isolation and no sound leakage. Very, VERY comfortable despite the weight and clamping force. Decently stable, but not good for active uses like jogging. Fold almost completely flat and are very easy to wear around the neck. Fairly high distortion in comparison to the others. Absolutely lacks treble, unfortunately, else these would rank much higher.
The rest: These rarely get used anymore, and thus have more brief descriptions.
6: Pioneer SE-700: Very flat and smooth response, no peaks or resonances or anything of the sort. Extremely low distortion. Unfortunately, even more difficult to power than the SE-500 and somewhat lacks speed and control. I know I still haven't heard these at their best, and unfortunately I might not ever, since the drivers are fragile and easily overvolted. These are one of the sexiest-looking headphones ever made.
7: "Pioneer SE-700RP" (T50RP drivers in an SE-700 shell): Pretty good, definitely a far better implementation than the stock T50RP shell. Flat sound with a slight tilt to bright. Fast attack, good decay, decent soundstage. High distortion is what kills these.
8: Stanton Dynaphase Sixty (modded): Extremely good bass and massive soundstage. A little muddy treble, probably needs new crossovers. Lacks control, gets congested on fast content. Decent speed despite the 2-way design. Double the weight and size of the LCD-2 (no, I'm not exhaggerating) but surprisingly comfortable.
9: Beyerdynamic DT1350: Good portables. Lacking in attack speed, good decay and control. Sounds very closed in. No soundstage is better than a closed in soundstage. Good treble and bass extension. Decent slam. What ruins these is a terrible haze in the midrange caused by the cup reflections.
10: Sansui SS-20: Another 2-way design. Very good detail retrieval, better than the Sixty and definitely uncharacteristic of the time period. Good soundstage. Sansui made some nice stuff. However, there's a pretty nasty resonance in the midrange that really just seems to ruin it for me.
11: Numark HV-215v: Decent headphones. These are a retro-DJ style can from the early '90s. As expected, lots of bass emphasis and shelved treble. Honky midsection and overcooked bass. Intriguing driver design. Surprisingly comfortable. Built like a tank.
12: Bang & Olufsen U70: Real stinkers. Sounds like a middier 4AAA with less detail and without any sort of bass slam. Very lacking in speed for an orthodynamic. Not all that comfortable. Very neat design and quite pretty. More of a display piece that just so happens to make noise.
13: KOSS K/6x Plus: My first headphones ever. Good detail, good treble and bass extension, but kind of muffled sounding. Infamous tin-can veiling so common in headphones from the '80s. Decent comfort and the famous tank-like KOSS build. Still somewhat listenable now.
14: Audio-Technica ATH-M50: Not so crazy about these anymore. Nasty sounding highs and a complete lack of control throughout the spectrum. Very farty bass without true extension. Closed-in sounding. Good build, but not great. My first "audiophile-grade" headphones.
15: Fostex T50RP (stock): Okay, so I technically don't still own a stock T50RP, but I do remember what they sound like. Even more middy sound than any of the others, and an incredibly muffled, veiled, distorty sound. Not very comfortable either. Really bad.
16: Audatron SH-608R: Nothing will ever top this, in stock form or modded. A Wal-Mart clone of the Numark HV-215v mentioned earlier. I used to like these quite a bit; I don't know what the hell was wrong with my ears back then. Worse build and FAR worse sound than the 215s. High distortion, overcooked bass, no treble, no soundstage, and uncomfortable as sin. No redeeming qualities whatsoever.