Slint

They are the godfathers of post-rock and if you consider yourself a music fan you HAVE to own Spiderland. It is one of the most accomplished, influential and shit scary albums of all time.
They pioneered the art of perfectly timed, complex time changes and used it to create two songs (Washer and Good morning captain) that put the complete works of any other band to shame. If you have heard Spiderland you will understand, if you don't then buy it!