Updated ranking, the ESP-950s and HP-3s are no longer in my collection, and I've added AKG K812, Sendy Aivas, and Grado GH3s. My ranking has become somewhat more convoluted.
1a. Hifiman HE-500 - My go-to for general music listening. Not a single weakness, and basically everything they do is incredible; large soundstage, smooth highs, slightly boosted, clean bass, and flat mid range. Eventually I'll start looking for a headphone to top this, but I suspect I'm going to be looking in the $2000+ range.
1b. Sony MDR-Z1R - I absolutely adore these, but some music can come off as too bright. When something plays to their strengths they are my favorite headphone. The bass is quite boosted, yet very clean and articulate; it makes them very fun to listen to for energetic music. Mid range is slightly elevated in the lower region, giving them a very full sound. Maybe not "reference", but it's absolutely intoxicating. Very large soundstage for a closed headphone, too. Actually, I might say they have a large soundstage, period.
2a. AKG K812 Pro - Tonal balance is remarkably similar to the HD800, to the point where it's hard to choose a favorite between them. The K812 has a smidge more bass, but the treble on the HD800 is a smidge smoother, so even there it's hard to decide which I like better. I really comes down to comfort and imaging, and that puts the K812 just over the HD800. The HD800 is so large it basically rests on your cheek bones, whereas the K812 is more reasonably sized, and fits like a proper headphone, rather than a helmet. As far as imaging, I prefer the way the K812 images. Soundstage is roughly the same size, maybe a little bigger on the HD800, but the HD800 to my brain seems to project the music mostly above me. The K812, by contrast, is more like a giant sphere of sound, where sounds transition from left to right more naturally than any headphone I've ever heard.
2b. Sennheiser HD800 - I expected these to give the HE-500 some competition, and I was right! They're similar in a lot of ways, though their overall sound tilts slightly to the bright side. I don't find them to be overly bright, though. It's just a very slight bump that can make them sound a little more... Metallic I guess? Still, a very musical headphone, despite what basically every review on the planet says about them. I don't find them to be overly clinical or sterile at all. I think they might have a slightly larger soundstage than the HE-500, but they image a little differently. I can sorta justify keeping both.
3. Beyerdynamic DT1770 Pro - I've crowned these headphones the king of bass. Very boosted, endless extension, and quite good impact. On top of that, incredibly fast bass. They're basically boosted like the Z1R, but fast like the HE-500. Treble is boosted, but I don't find it to be very grating. I think because it's a broad emphasis, and not just specific frequencies, it comes off as more musical. One of the things that make these so impressive is that they are very lively at low volumes. You can basically jam out at whisper volumes and save your ears a beating.
4. Sendy Audio Aiva - A well-balanced, though somewhat lean sounding headphone. The lowest regions of bass leave something to be desired, but there's good bass impact. Treble is generally on point, but there's a bit of an upper-treble emphasis. It gives these a unique, live sound, but it doesn't always sound natural. They image really well, I'm assuming in part because of that upper-treble spike, and soundstage is of decent size. Mid ranges is good, if somewhat unexceptional. The overall sound is somewhere around reference, leaning toward the fun sided.
5. Grado GH3 - An absolute blast of a headphone. Not the most refined sound ever, but they lavish in joy so much it's really hard to complain. Treble has some odd dark spots in it, as well has some pretty significant spikes, and mid range seems to have a bit of a lower-mid emphasis. Bass hits HARD, though extension is weak. It all adds up to a very visceral, lively sound that's really hard to top. Soundstage and imaging are fantastic, especially for an on-ear headphone. I don't think Grados should ever be someone's sole headphone, but I think everyone should own a Grado.
6. - AKG K612 Pro - One of my favorite sub-$200 headphones. They're so well-rounded, there's not really a single thing they don't do well. Bass is well extended, treble is right where it should be, mids are pretty much unmatched, soundstage is huge, and imaging is fantastic. I used to complain that they are hard to drive, but amps have really come a long way since the K601. These never really got the love I feel they deserve. I think they are every bit the equal of the likes of HD600/650/6XX, yet come priced well under.
7. Sony MDR-CD900ST - The most detailed headphone I've ever heard, by a country mile. Before hearing the HD800, I thought that might be the one to beat these in that regard, but nope, these are still the detail king. Bass has good impact, though weak extension. Treble is subdued, so they generally sound laid back, yet they reach into the music and pull out every last detail there is. As I've heard more headphones, I've found these to be less and less musical, to the point where I don't really reach for them all that much anymore. I've come to realize that these really, truly are a studio tool.
8. AKG K501 - I like these well enough. They're remarkably similar to the K612, though lack the bass quantity and extension. They're a neat novelty from their time, but don't really stand up to modern headphones.
9. Ultrasone HFI-580 - A curious, yet underrated headphone in my book. It's probably the most traditional sounding headphone by Ultrasone, in that it has the weakest S-Logic effect. It still has it a little, you can tell by the fact that the soundstage is quite large for a closed headphone, but it doesn't have the weird imaging that the other S-Logic headphones have. Treble is fairly boosted, though not to the same degree as their other headphones. Mid range is flat, which actually makes them kinda impressive. At the time these came out they were fighting with other bass-boosted headphones, but most of those has messed up mid ranges. The Sony MDR-XB700 and Audio Technica M50s in particular come to mind. It doesn't quite have the great extension as those two, but the impact of the bass is really something to behold.
10. Beyerdynamic Aventho - My beater on-the-go headphone. Good sounding, though a little bright. Bass presence is a little weak, but not terrible. They have decent imaging, though small soundstage. Not really a whole lot to say about them. Actually, the rest of my headphones are going to follow this trend.
11. Sennheiser HD-25 - My previous beater headphone. Again, sounds good, but too bright. Moving to the Aventho cut down on some of that, but these two are roughly equal in my book.
12. Koss Porta Pro - Very dark sounding headphone, but still enjoyable. Nice bass impact, solid mid range, and pretty much anything sounds good through them. Soundstage and imaging are lacking, though. Still, not bad for $40 or whatever.
13. Sony MDR-XB700 - Sort of a train wreck, sort of impressive. Mid range is absolutely garbage, and treble is a little bright, but the soundstage and imaging are decent for a headphone of this sort. Bass is very boosted, and is absolutely bottomless. It's very clean, though. You can basically EQ unlimited bass into these and they don't break up.
