To my ears, from my experiences to date, these are my choices for "best" available. [Subjective mind/ear interface POV]:
[size=large]#1 - Sennheiser Orpheus System: HE90 phones + HEV90 (dedicated) Amplifier[/size] - Headfi Nirvana! The HE90 alone with other amps is excellent, but does not match the unparalleled experience of the complete system.
[size=small]Qualia 010[/size] - The most complete sonic transducer, capable of reproducing the subtlest, most complex components of each sound with great extension, air, dimension, rich timbral presentation, definition and placement. Very efficient and easily driven, but requires the best ancillary components and best quality recordings. It is more sensitive and reactive to inadequacies in every step in the sonic chain than any other headphone I'm aware of. Near-zero isolation: lots of sounds comes in and leaks out. These phones are people-polarizing, and they are so demanding and finicky that if you've only heard them at their worst (imperfect fit, imperfect recordings, imperfect system synergy, noisy surroundings, etc.), you'll never know how great they can sound ... sometimes.
[size=small]AKG K1000[/size] - "ear speaker" or not - they sound beautiful. They create a huge, open, immersive sound space and a sweet, lucid, extended sound. It lacks the relaxed comfort of the R10s -- or even the 501s -- but it provides a great musical experience. Especially sweet with the Dared Monoblocks, but perform beautifully with a variety of amps. They may be the most power-hungry phones.
[size=small]AKG K501[/size] - My practical favorite to date, since I can't yet afford the Orpheus system, or Qualia 010 with necessary uber-system. Tight and well-integrated bass, beautiful midrange and fine detail overall. The 501 does reproduce deep bass frequencies, but not usually with the robust impact of some other phones. The GSP Solo 2004 -- as PinkFloyd has exclaimed -- has the power and signal quality the K501 needs for "bags 'o bass" and a big, dynamic sound (tho' modding this amp for Grados apparently kills its synergy with the 501s). They also completely surround the ear with room to spare and "breathe" so well that they're almost as heat-free as the no-contact K1000. A brief listen with Ray Samuel's Hornet (tiny portable) headamp + Headroom MicroDAC produced a huge sound out of the 501s. Since the Hornet is not yet in production at this writing (9/21/05), the effect of the final production units on the 501s remains to be seen. SR-71 works beautifully with the 501s and creates and great, spacious, lucid sound with lots of "air" around performers, fine timbral nuance, ultrafine detail. The portaphile PV2^2 handles the 501s admirably in different ways, adding body, impact, low-end punch and a lively & forward presentation. The MicroDAC is a great "source" for these (and most other) phones, adding great dynamics and texture.
[size=small]Grado HP1000 & HP2[/size] - After a few short sessions with these I find their sound very much to my liking, but haven't yet found a pair that fits me comfortably. Need to find sets that fit better and hear them with more systems to form an accurate personal placement.
[size=small]Stax Omega II[/size] - Great sound with KGSS amp via Meridian G08/Meitner DAC. Lots of fun, though overly huge bass and ultimately not the more balanced sound I prefer. Waiting to hear them with immtbiker's new EMMLabs CDSD/DCC2!
[size=small]Sony MDR-R10[/size] - A grand, comfy plush armchair of a headphone. Seductive and relaxed with big body, bass and punch - a great vintage cabernet of a headphone. I like the R10, but I sometimes find the sound is too much "in your face" with some recordings - and doubt I'd spend $4k if I had it (tho' if I had cash to burn, I'd buy all the toys just for the fun of it).
[Always more to come. Soon to audition: AKG K601 & K701]
It's pretty obvious that my prefererence is for a certain well-defined sound, judging by my AKG K1000/K501/Qualia010 choices. HE90/HEV90 is my "choir of angels" ultimate choice (and the HEV90 amp is a MOMA-worthy visual experience...).
{we need an icon maestro to create more smileys!}