It's a very difficult task to really come up with a good, or even acceptable answer to this question. I could relay my humble experience to you, but at the end of the day what determines the best-cans in production isn't popularity but your own personal preferences. No can is perfect, no new can is even good at 60 percent of things, thus if you are truly looking for the perfect headphone all I can recommend is that you go to different meets and try stuff out.
The Stax Omega II
The stax OII starts at around 1500 dollars. The amp I got to try this out (LA meet, a year ago) was a 4500 Singlepower ES-1. From what I got out of this, I can testify to this can's performance brilliance. It reached higher and lower than any other can i've heard yet. It has a sparkling and beautiful treble. The bass is very well controlled, articulated, "tight" and dry sounding. There is a story that the can was developed to let motor-engineers hear the details of engines, While I don't know if this is true, I would certainly believe it after listening to this can. Beyond this, the soundstage is wide, the resolution is the best in the hobby, and it can convey a great orchestra like no other can. If there is a problem, it is the "problem" it shares with all other electrostatic cans: they do not convey a sense of "being there" the way that the best dynamic cans do, in addition the bass is good, but not quite as satisfactory as an L3000 or Edition 9
The Ultrasone Edition 9. I got only like 30 minutes with this can from a Corda Opera and Gilmore Balanced Reference. I would say it is the quickest, most authoritative can I have ever heard. The soundstage is somewhat nil (to a person who likes K1000/OII like soundstage), but It doesn't really matter if your listening tastes don't include Gustav Mahler. Iron_Dreamer said it has a "sick" bass, and this is certainly true. The sheer texture, reach, power, tone, etc... of the bass makes the listener believe that they are listening to the mixer... the HD650 by comparison sounds rolled off and "one note bass."
the HD650
I've heard it in everything there is. The can "scales like a bitch"- I cannot really put it better than that. When I first heard it, it was nothing really, just a big PX100 out of my ipod. Yet, as I've great heard system after great system I have noticed how much the sound quality of the can can improve as you add better and better components up the chain. Even the cable can make an enormous difference. I've said it before, and I'll say it again: the HD650 is the best all around can in production. It won't give you the best bass, treble, soundstage, tone, neutrality or whatever.... but It won't make any mistakes. You can make it into a lush-beautiful/heart-warming headphone that can go toe to toe with the HE90, or you can make it a bright/fast,lean headphone capable of competing with the OII. the options are really limit less, As i've heard different interpretation after different interpretation of the HD650, I always feel inspired to try something new.