Overall ranking (I listen mainly at home, classical music):
1) Hifiman HE-6: Neutral sounding, very good detail. One single drawback if you already have a headphone amp setup: a bear to amp well and it does best with a low-power, low noise speaker amp. Not very comfortable and it's heavy.
2) Sennheiser HD-800: Extremely detailed and resolving, very comfortable, relatively neutral, treble sometimes a bit strident. Drawbacks are the price and the fact that the detailed nature becomes a disadvantage if source quality is not optimal.
3) Hifiman HE-500: 90% of the HE-6 at half the price (1/4 the the price if you count the amplification), can be driven with a normal headphone amp. Very good midrange and vocals, sounds a tad on the warm side. Ideal with simple string music / vocals / jazz, not optimal with complicated works where there's a lot going on. Cons: not as detailed and resolving as its brother, sometimes a slightly veiled sound impression.
No longer with me (sold):
Beyerdynamic DT-880 in 600 Ohm edition: Very good bang/buck, relatively detailed, 600 Ohm version plays very well with powerful tube amps. The problem: common with all Beyers I don't like its treble presentation, find it a bit on the harsh side. Had to go as I already owned 3 headphones and this was the least of the bunch to me.