(Added TH909, ATH-R70x. Reordered some things. Added a second recommendation level.)
+++=Highly recommended
+=Recommended
Recommendations are based on overall satisfaction including price.
I've owned all of these.
Tier 1 - The best I've heard
+++Fostex TH909 (Open version of the TH900. Slightly improves on soundstage and openness while keeping the best dynamics.)
+DCA Stealth (Jack of all trades, master of most. Good isolation, excellent imaging, good soundstage, excellent clarity, best bass extension (audible down to 13 Hz or so!), best tuning of any headphone ever, very good dynamics (on a Topping A90 Discreet), good comfort, excellent customer service, excellent cable, excellent cable attachment mechanism, excellent design.)
+Hifiman Susvara (Wider soundstage than Stealth but no isolation. Imaging/separation great but not quite as good as Stealth. Has excellent tuning, dynamics, and soundstage. Comfortable, good build quality.)
Tier 2 - Almost the best I've heard
+++Fostex TH900MK1 (Best dynamics I've ever heard along with the TH909. V-shaped signature, very clear, awesome bass. Closed back, but leaks a lot of sound. The most fun headphone I've heard.)
+++Fostex TH900MK2 Anniversary Tuning (Like the original, but with more balanced sound. Bass and treble lose some impact though. Good if you find the original too bright.)
+DCA Voce (Better ergonomics, cable, ear pads, headband, and customer service (God-tier vs. non-existent) than Stax. Also much reduced "Stax fart" compared to the 007/009. A little less treble than the 009 but with much more balanced bass performance. Similar transparency and bass impact. Requires a good electrostatic amp.)
Stax SR-009 (Has increased imaging and separation over the SR-007. Dynamics seem slightly better. Unmodded has "Stax fart" similar to modded SR-007A and I assume the original MK1.)
Tier 3
+++Sony IER-M9 (A little uncomfortable. Sounds nearly perfect if you can get the fit right. Has more lower-midbass than Z1R which increases impact.)
+Stax SR-L700MK2 (VERY similar to Focal Clear. Has slightly more mid-bass impact, midrange is not quite as thick. Instruments seem slightly more separated. Soundstage slightly wider. Very similar to SR-007. Soundstage is a little wider on the SR-007 and it has more lower bass until you do the Blu-Tack mod which evens it out.)
ZMF Verite Open (Kind of like a smoother, more musical estat with excellent bass. Pads are easy to swap and good quality. Nicely layered sound. Make sure rods are tightened or you can lose an eye.)
Sony IER-Z1R (A little uncomfortable. Sounds nearly perfect if you can get the fit right. Could use a little more bass impact.)
Stax SR-007MK2 with Blu-Tack mod (like the SR-007A but with less "Stax fart" for some reason.)
Stax SR-007A with Blu-Tack mod ("Stax fart" a little annoying.)
Hifiman HE1000SE (Detailed sound with good bass and transparency. Almost electrostatic-like speed.)
Sennheiser HD800S (Super transparent with more bass than Stax SR Lambda Pro. Lightweight but ear pads are large and don't conform well. Great soundstage.)
Tier 4 - Beginning of summit-fi. Can be considered end-game at this point for most.
+++Audio-Technica ATH-R70x (Cheap! Like a baby TH909. Excellent sound signature, bass, and openness. Falls short of the TH909 in just about every category, but excellent value and all around performance. Requires even more amplification than the TH909 or ZMF Verite, expect to spend at least $350 on an amp. Very light weight. I actually prefer this to all their high-end models.)
+Focal Clear original, not MG (Not the widest soundstage but excellent tonality, dynamics, and extension.)
+Focal Clear Professional (Same as the OG. Not the widest soundstage but excellent tonality, dynamics, and extension.)
+ZMF Eikon (Similar to the MDR-Z1R but with more even frequency response but less bass extension. Make sure rods are tightened or you can lose an eye.)
ZMF Verite Closed (Sounds similar to the Verite Open, but with a very slight muffled closed-back sound to it. Make sure rods are tightened or you can lose an eye.)
Sony MDR-Z1R (Closed headphones with great transparency and better bass than HD800S. Amazing for a closed back. Similar to ZMF Eikon but with slight peaks in freq. response that sound a little unnatural at times and better bass extension.)
Audeze LCD-XC 2021 (Good sound quality. Closed back. Has treble slam but a little lacking in bass slam. Heavy. Doesn't fit my head without modifications. Bass needs EQ.)
Tier 5
+Koss ESP/95X (Very natural sounding with wide soundstage and great transparency. Mid-centric. Fantastic for easy-listening and voice. Bass and treble could use a boost. Lightweight and comfortable.)
Meze Empyrean (Very comfortable, outside mesh not durable. A bit dark sounding and lacking detail.)
Focal Utopia (Slightly wider soundstage than the Clear. Sounds very similar. A little too thick sounding for me resulting in a "wall of sound" and less instrument separation.)
Nectar Audio Hive (Similar to Stax SR-009 except is more forward sounding and not quite as "ethereal". Slightly more bass impact. Requires at least 400 V RMS energizer. Prone to problems with dust in drivers.)
Sennheiser HD820 (Comes close to the HD800S but isn't as tonally even and soundstage is slightly smaller. Has better comfort, isolation, and bass volume though.)
Tier 6
Focal Clear MG Professional (Not quite as clear sounding as the original. Slightly warmer. Soundstage seems smaller than original.)
Hifiman Arya (A little lacking in dynamics, sounds a little flat. Soundstage could be wider.)
Campfire Andromeda (Rivals HD800S / MDR-Z1R in transparency and very even sounding. Good everything, except bass is a bit light. Sound stage is a little smaller than with over ear headphones.)
Audeze LCD-4 (Best bass, good but not great transparency, very heavy, prone to driver failures with pressure changes. Mine had a driver failure.)
Tier 7
+Audeze Mobius (Some of the best bass I've heard but soundstage is small.)
+Razer Opus (Very good timbre and frequency response. Treble and bass are could use more clarity.)
+Sennheiser HD600 (Excellent value. Bass extension is a little lacking. Excellent build quality.)
Sennheiser HD650 (Like the HD600 but very slightly darker sounding and a little less natural treble.)
Kaldas RR-1 Conquest (Has great bass extension for a closed back. Prone to mechanical issues.)
SendyAudio Aiva (Great build quality, looks good, good frequency response except bass extension is lacking.)
Stax SR Lambda Pro (Super transparent.)
Audeze LCD-2 closed back (Heavy, bass extension and impact are lacking, isolation is poor for a closed back.)
AKG K7XX (Great value. Elastic may go bad over time.)
Tier 8 - Beginning of hi-fi
+qdc Neptune (Good value, sounds like the Andromeda, but a little more muffled and more bass and bass extension).
Samsung AKG EO-IG955 (Surprisingly competent. There are a ton of knockoffs of these which sound awful.)
Grado SR-80e with L cushions (L cushions improve sound quite a bit, kind of uncomfortable, tons of midbass, good transparency.)
Beyerdynamic DT880 (Annoying treble peak, ~8.5 kHz.)
AKG K702 (Painful headband.)
Beyerdynamic DT990
Superlux HD681 EVO (Excellent value, well balanced sound. Plasticky build. Bass is a little bloated sounding).
Tier 9 - Pretty good
Denon D2000
Shure SE846 (Terrible value. Sounds similar to the IE80.)
Sennheiser IE80 (Good value, but sound a little muffled compared to higher end stuff.)
Etymotic ER4XR (Good as earplugs. Sound not very impressive.)
Sennheiser HD598 (A little muffled sounding.)
Bose Triport (Good 3D sound positioning. Lightweight. A bit muffled overall.)
Koss Porta Pro (Very light weight. Sound is good for price. Open back design.)
Tier 10 - Decent
+VJJB-V1S (Sound is very good for price.)
+ViSang R02 (Sound is very good for price.)
Westone W30 (All mid-range. Not very good for listening to music.)
Grado SR-60i (Poor fit, comfort and bass. Decent transparency. Would probably sound better with L cushions.)
KZ ZS6 (Treble is peaky.)
Tier 11 - Acceptable
Panasonic RPHJE120S
Panasonic RP HT355
Tier 12 - Bad
Samsung AKG EO-IG955 Knockoff (Bought for about $4 and returned it. The legit ones are about $100. Sounded like music through a tin can.)