An OP I feel is really overlooked is the one for Ayakashi: Japanese Classic Horror (a.k.a: Samurai Horror Tales). Technically this show has three versions of the opener, but this version is my favourite. Love the mix of traditional Japanese imagery and music with all the psychedelic patterns and hip-hop stylin'.
On a related note: the part of Ayakashi that this OP comes from was the precursor to Mononoke, which also has a great OP in a similar visual style. Not as big a fan of the song, though.