Overrated; LCD2 - I think its a great headphone and its one of my faves, but it is very limited with genre. When it first came out peeps raved and raved about it being the best thing since sliced bread. It has a very much love or hate sound.
Underrated; HD600 - One of the oldest headphones on head-fi and probably the best value for money. Of all the so called advancement in technology over the last twenty years it still stands its own as a brilliant and very versatile headphone. It also has the most natural tonality that I've heard below the HD800.
I wouldn't say that the HD600 are underrated. They still get plenty of love and are being recommended