I read this entire thread and can't believe there was only 1 mention of the Beyerdynamic T70. The T70 is unique because it really can compete with the best of the best imo. I like the neutralish/bright sound signature, however I swapped the stock earpads on my T70 for T5P one's and now the sound is completely neutral..more neutral than even the Sennheiser HD600..there's not even a hint of coloration to the sound.
A few other headphones I consider underrated;
Meze 88 Classics; Natural, well balanced sound with an impressive soundstage.
Pioneer SE-A1000; One of the most musical headphones I ever heard.
Panasonic RP-HTF600E; Has powerful 50mm drivers that produce deep but accurate sub-bass, clear mid's and clear high's.
JVC Esnsy; Possibly the best $30 - $50 headphone I ever heard. Powerful but accurate sub-bass, warm mid's and clear high's. Realistic 3d soundstage due to having angled drivers.