This is a difficult question...
Head-fi, which pair of headphones (of the ones you currently own) do you keep if you only get one, and WHY?
The circumaural HifiMAN HE-6. But the supra-aural Martin Logan Mikros 90 portables are--to my nearly dumbfounded surprise--not too far behind! : )
HiFiMan HE-6 here also, for me the best presentation of sound to my tastes (over the others I've owned/still own being the HD800, T1, GS1000i, LCD2.2, Ed 8, Signature Pro, T5p, W3000ANV, HD25-1 II, HE-400 and Q701).
If I could have afforded to keep my Stax SR-Lambda with the stock drivers (not the rebuilt ones in my other set with noticeably less extension both ways), this would be a very easy question to answer.
But I didn't, so...this is a tough call.
On one hand, there's the Sony MDR-MA900, with its ludicrous soundstage, great mids, unparalleled comfort and high efficiency.
On the other hand, there's the Beyerdynamic MMX 300, with its more substantial build, greater isolation and lack of leakage, superior treble, and a built-in mic with surprisingly good noise-cancelling qualities and clarity.
Neither have the mind-blowing clarity and reproduction of the SR-Lambda, though. That headphone's just in another class from everything else I've ever used, particularly in midrange/vocals...but so are the amplification requirements if you don't have a speaker amp and transformer box handy.
Ultimate Ears Reference Monitors. Sounds amazing out of any low impedance source, and is as portable as it gets. Scale sup very well with gear as well. Super comfy and very portable; blocks out noise while im outside.