My picks have been mentioned but I'll add my thoughts anyway.
Overrated: M50 hands down. At their Retail point $150 they are outclassed by a few lower models. They are decent at $100.
They became over rated by the Beats Crowd. Look at the old reviews and most people said average sound etc. But the Beat guys found out they were better, then they became the best thing ever. No they just sound better then Beats. Entry level headphones.
Bad comfort, pesky highs, sucked out Mids. No soundstage...Build is great tho.
I would add the 650 as well, BUT it's just personally I don't like them. I think they are just a tad overpriced..$350-400 (New) should be right.
Underrated. Again 240s, A great headphone. And you can get it CHEAP when on sale. As low as $50 sometimes. 271 as well, another great set. Both of these had fans a few years ago, but the headphone crowd has changed. Now it's BASS BASS BASS. if it has no Bass no one even gives top end (Cheap) a try.
HD428, yep @$24 it's hard to beat. HD439, another fantastic set, Again it doesn't have that booming BASS, so it's overlooked.
Great comfort, removable cables etc...$100 is about right on the price. At it's point it fits in nicely. Better sounding then the M50 IMO, Different sound but more my taste.
I'll agree on the DT770 as well. There are many fans as these have been around forever. Comfort, Build,Soundstage for a closed can (Even better then most open phones) And price. If you shop around you can get them shipped for $150. You side by side these with the M50 and you see just how bad the M50 sounds. I'll say tho, that different pairs don't sound the same. I believe production runs have changed. I had a set about 5 or 6 years ago and they sound nothing like the set I just bought.
The old set had MASSIVE bass, A sucked out Mid (Hollow sounding), and very harsh treble. The new set is so different sounding.
The same hard hitting Bass, but the Mids are in Balance, & and the highs are crisp and smooth (Almost Seen like). I'd almost call them Balance sounding. Not quite but almost. I think you really have to go into the mid 300-400 range to find a true upgrade.
A fantastic set of headphones. You know the post's if you could only have one headphone? The DT770 is it for me. I could live with it and be VERY happy.