Comfort is always fairly undervalued but that's also something that is very user specific.

I think Soundstage is the most overvalued (me a 6X0 enjoyer)

Soundstage is overvalued... Good imaging, timbre and tonality trumps every "perceived" soundstage... But all this goes down the shitter if the headphones feel like torture devices on one's head

I absolutely adore good imaging.. but not at the expense of tonality.

I’m ok fine-tuning imperfections, but all the “technicalities” in the world aren’t going to make me love some overpriced headphone that needs major EQing just to not sound like total ass tonality/timbre wise. Some very expensive headphones get away with murder in this department.

Everything is important. It doesn't matter what it is that makes it enjoyable. The point is that there has to be something that makes me want to use the headphones and at the same time I shouldn't be annoyed by something else too much.

But if by transience you mean dynamics I have to agree that it's often overlooked. A good example of this is Thieaudio Phantom which is a pretty terrible headphone overall but it does dynamics so good I still keep them as a benchmark. Another examples are DT900, HD660S, or K712. People often say that they're not worth it because of DT880, HD600/6XX, or K701/612/702, completely ignoring a huge gap in the dynamics. On the opposite, this is also the case with Hifiman's non-round open-backs which on the opposite lack the dynamics and people often ignore it, saying for example that Ananda is the straight-out upgrade from Sundara which isn't true.

From what I know you should have little to no soundstage and as much imaging as possible: Soundstage should be defined by the recording (if the said recording even wants any soundstage) while imaging is required to better play the said recording.

Basically passive soundstage bad because not all recording are supposed to be wide, good imaging good because it represent the recording better

imo it’s tonality mainly because some people will say “just use EQ” in response to anyone asking about it

Most overvalued is soundstage. I can barely tell a difference between headphones when it comes to staging (yes even between open backs and closed backs). You’re paying for like 1-2 extra inches of staging when you get an HD800s

Soundstage is the most overvalued because headphones do not have any soundstage at all.

Timbre is the most undervalued because people tend to prefer "technicality" over it.