I've found most high end headphones to be overrated - frequency response oddities and high bass distortion seems to be the norm (the latter mostly for dynamic transducer headphones). Questionable build quality and/or comfort are also too common in high price ranges.
But what takes the cake for most overrated for me are Audeze headphones, including the LCD-4 I briefly owned as well as the LCD-2, LCD-3, and LCD-X. They all have a massive upper mid/lower treble valley that just makes them sound broken to my ears, I can't fathom how anyone can enjoy them. On top of that, they're so heavy. At least they produce really low distortion levels and phase issues are only typical, though so EQ can mostly save them.
While not rated as highly in the West, I can't understand why Audio Technica still make the sorts of expensive headphones they continue to make. It seems the more expensive models just get wonkier and wonkier. The ATH-W1000Z is one of the absolute worst headphones I've ever listened to, too many FR and probably phase issues to count.
I also find the popular Fostex TH-900 to sound absolutely awful. The mid range sounds like it's under water.
I strongly agree with the Sennheiser HD 800, and HD 800S, being named above. They have a less linear sounding frequency response than the HD 600 series, HD 560S, HD 490 Pro, and don't fix the bass distortion problems they have so why should anyone buy one?
As for underrated... although popular here I'll have to name the AKG K371 and the 7Hz IEMs that measure wonderfully like my Crinacle Zero:2. These are very affordable, and when listening to them it's hard to imagine why anyone would spend thousands on a headphone, until I switch to my DCA E3 that is. But even then, the K371 delivers so much for such a good price.
On that note, I may have to add the DCA E3 to my underrated list because I don't think it won any audiophool awards last year and people seem to be forgetting about it already. Perhaps Dan Clark should've made it $6k and added more snake oil to the marketing - then I bet they'd still be talking about it (positively).