For me, a great midrange presents no veil over the vocals, no texture, and isn't totally stepped on by the bass and treble.
At least, that's what my limited experience with a few headphone models has taught me. It became most obvious to me when I got my first Stax SR-Lambda, when there was suddenly this sense of clarity beyond what everyone else had described and it seemed like people were singing out to me, and then when I got an SR-202 setup afterward, where the vocals suddenly took a noticeable back seat and EQing up the midrange revealed this sort of "grain" in the audio that shouldn't have been there.
I couldn't rule out the amps due to the bias voltage differences and lack of a dual-bias amp, but it did show me what I really wanted out of a headphone more than anything (alongside general gaming prowess with headphone surround mixes.)
I know, it seems shallow to judge the midrange entirely on vocal presentation, but that's the easiest way for me to notice it. I suppose the other way to notice it is to listen if high-pitched or low-pitched notes aren't completely overshadowing everything else, if that's what the source calls for.