This is a tough one and subjective as hell (isn't everything in our perception of sound?), but here goes:
OVERRATED
-- Audio-Technica ATH-M40x and M50x. I've seen these cans recommended by many as an introduction to audiophile headphones. Well, anyone who makes that recommendation must really hate you, as both of these cans are TERRIBLE. V-shaped as hell, with bloomy bass, scooped mids and hot, crispy treble. Also the most uncomfortable headphones I've ever worn.
-- Philips SHP9500. Another entry-level can that's recommended as a first open-back. No. Just no. No bass, all treble. Incredibly loose fit, even for big heads like mine. The imaging is solid -- the only thing going for this godawful headphone.
-- Sennheiser 560s. Recommended by many as an entry-level paragon of neutrality. The treble peak is anything but neutral. I know it probably hits me in just the wrong spot and doesn't bother others. But the other problem with this headphone is its lack of colorization other than the treble peak. I'm sure they're fine for mixing and see people praise them to the ionosphere for gaming, but they're just boring as hell for music.
-- Dan Clark Audio Aeon 2 Noire. Bought these. Sold them within two weeks. Blunted treble, hard to drive, pads that roll on themselves and excessive cabin pressure due to a tight seal were killers for me. Did not like these at all and don't understand the hype or the price.
UNDERRATED
-- Meze 99 Classics. I can see the eyes rolling now. Hear me out. The 99 Classics might be the ideal gateway drug into audiophile headphones for those accustomed to consumer wireless signatures from Bose, Sony, Sennheiser and Beats. In fact, I call the 99 Classics "Beats in a tuxedo." Tons of bass, but OK mids and surprising resolution in the treble. Plus they're easy as hell to drive -- straight from a phone -- and are well built and beautiful. You could give a novice audiophile coming from Best Buy over-ear wireless cans much worse advice than to drop $300 on 99 Classics or $200 on 99 Neo.
-- HiFiMan HE-400se. This is one of the best values in all of audiophilia. A superb entry-level, open-back planar. Brilliant at nothing, solid at everything. And an insane value at $110ish.
-- AFUL MagicOne. My daily driver IEM right now. Such a unique tuning -- with only one balanced armature driver -- amid a sea of me-too Harman-tuned IEM releases every week. Lovely tone and timbre. Much more of an emphasis on musicality and cohesion that microscopic detail. Just a joy to listen to for extended periods. I got them for $110 with a coupon at Amazon. HiFiGo is selling them for $99 now as part of the March sale. A total steal. Love 'em.
-- Focal Stellia. The best headphone I've heard, period. And since it's been on the market for about five years or longer, it no longer gets the same kind of respect as the past as a TOTL closed-back can. The Stellia did nothing wrong during my testing. Didn't want to remove them. I liked them even better than the original Utopia and a hell of a lot more than the HD800S (yes, I know those are open back).
I wanted to put the Focal Elegia and Audeze LCD-X 2021 into the overrated class but couldn't do it. I owned and enjoyed both -- with EQ. Without EQ, the mids' tuning for both is so f*cked up that I can't imagine listening to them without extensive parametric EQ fixes. But both are pretty nice (Elegia) or lovely (LCD-X 2021) with EQ.