I've listened to many headphones from various manufacturers, but I stopped looking when I heard the HD-800.
I've rarely heard a pair of headphones that scale so effortlessly, given the source. It sounds very even and likable without amping - even from an iPod. But when amped, it's remarkable even controlling for the music, from pop and rock to lo-fi punk.
(Why "Sennheiser HD-800 Appreciation Page, v.2"? The original thread has been archived after over 6600 posts - but there's still a lot of interest in these cans.)