Anything from Asian Kung Fu Generation. And not just the OP's they did for FMA, Naruto and Bleach: they have an insanely sick library.
When your playlist includes AKFG, the Pillows (FLCL), and the Beat Crusaders (Beck), there's an emptyness you feel when a song doesn't make you headbang 160 mph and sing along in a language you have no business singing.
And then you have the song from
Azamanga Daioh's OP. Words can't describe how illogically addictive it is.