K371: to get a good idea about what the Harman target curve sounds like without having to get into EQ. And if you don't like it, it won't hurt you too much financially when you want to re-sell it.
It varies enough because of sealing issues that I'm not sure that what a lot of people are hearing with these is representative of Harman's intent with their over-ears target.
https://www.rtings.com/headphones/1-4/graph#1671/4007
https://diyaudioheaven.wordpress.com/headphones/measurements/akg/k371/
There may also be quite a bit of channel imbalance and sample to sample variation with them.
We have a small dozen at work and I also have my own pair (the latter of which I've tried to measure on my own head with in concha mics below 1khz). The ones at work which pads have been crumpled because of poor packaging design I just can't even use as I get no seal with them. My own copy also has fairly asymmetrical pads and I'm struggling a lot more with them to get decent channel matching on my ears than with other headphones.
I think that EQing a pair of HD6XX/650/600 to Harman is a better bet if one wants to hear what the target was meant to sound like... and even then pad ageing may throw off the results.