HE90 for me but I've always been an electrostat kind of guy, speakers included. But it's close. They are my favorite 2 headphones, followed by the K1000.
The HE90/HEV90 is about as good as it gets, although I'm told that the ES-1 takes it a little further, but I'll just have to keep waiting for Mikhail to get around to finishing mine. It just does everything so effortlessly, so crisp and clear and open sounding. I can't really find fault with the Orpheus system with a top notch source.
The R10 of course has the seductive mids and an uncanny ability to extract details without making them stick out in the presentation like out of place (i.e., the extra detail of the R10 relative to other dynamic cans don't come across as oddities or artifacts). Everything about the R10 presentation is so "musical"; they just suck you into the presentation.
But then again I could say the same thing about the HE90. I think the main difference is that the HE90 wins on overall crispness whereas the R10 wins on the overall romantic, alluring factor. So it depends of which you're more drawn to and opinions will obviously differ.
There are people who prefer the L3000 to the R10 because of the added bass response and PRaT factor that the L3000 brings to play. Not as romantic or euphonic as the R10, but has more pop and sizzle, and thus a better (more impactful and awe inspirng) pair of headphones for dynamic swings and that sort of thing.
This discussion could then lead into the PS-1 for it ginormous and untra clean and coherent bass response (talk about holding a deep note through the last moment of decay!), or to the Qualia 010 for their ability to give you a microscopic view of the inner detail of the music and absolutely lightning speed, or to the K1000 (properly amped, which isn't easy) for an obviously unmatched soundstage (its kind of an unfair fight there) and nearly as much quickness as the Qualia 010 and nearly as much crispness as the HE90.
So it's not as though the HE90 and R10 are in a league of their own, though I would agree that generally speaking, they are widely regarded as being "best of class" in their respective categories.
I've never really heard the R10 balanced, but I did hear the HD650 balanced on a Singlepower SDS-XLR several years ago at a meet in Florida, and then again in New York, and the HD650 are completely different animals when run in balanced mode in the SDS-XLR. So by extension, I'm expecting the same to be true of the R10 balanced on the SDS-XLR but of course I'm also still waiting (forever and a day) for my SDS-XLR. So I don't know... it's getting to be quite frustrating, but that's life I guess.
My R10 listening has been mostly on my Angstrom Research Two Box (a one off amp that I bought from Nik). It's bar none, by far, the best single ended dynamic headphone amp I've ever heard and it really brings the R10 into full life. I know from my detailed comparisons of the HD650 in single ended versus balanced when run off the same amp (SDS-XLR) that the R10 can and will get much better when run off a off top notch balanced amp, but that too remains to be seen.