I haven't used enough products to give a full list. I've also used a bunch of buds too which I view in the same category as IEMs, just trading out isolation.
I feel soundstage is very important simply because it is hard to "fix." Frequency response can be EQed, within some limitation. Creating a stage presence with exact location, separate, and physical size are things much harder to adjust.
Bass, Mid, Treble as a whole:
1) Triple.Fi 10
2) Yuin OK1
3) Head Direct RE0
These where the three I've used where it is very hard to put specific fault on frequency response. None of these earphones sound alike, but they all don't fall short or really mess up in any of the 3 areas.
I'd have a much harder time listing individual regions. You start ignoring the fact that a headphone does terrible somewhere else. I mean, what's the point of great mids if the high or low frequencies are horrid?
Sound stage: pinpoint location, width, and depth as a whole:
1) Etymotic ER4S
2) Denon C751
3) Yuin OK1
From a total mental image standpoint, these three mainly stand out for providing good location, separate, space, width, and depth
I'll add a third category simply because I think it's probably the most important factor of all: realism. What I mean by this is can the earphone fool you? Is it so good that noises, talking, etc. actually fool you into thinking they are real? Does it sound life-like? I kind of feel this is the desired end goal, is it not?
Realism:
1) Yuin OK1
.
2) Denon C751
3) UM3X
The OK1 is really the only one that can truly fool me, and it's happened several times. No other earphone that I've used has done this. The Denon C751 is very good in the sense that the sound actually has a life-like presence to it, but it's bound more to just the midrange frequencies. Something like the UM3X is good, but it doesn't quite convince that it sounds like the real world although natural, even, and dynamic in presentation, but they sound more like a recording of real then as real. They're more like listening to a good set of home theater speakers. They sound really good, but there's always that sense of speaker between you and the source. Something like the TF10 is better in you hearing the source directly without the sense of speaker between your ear and the original material, but the TF10 isn't life-like with a more analytical/synthetic coloration, real through colored glasses.
