My Bloody Valentine, especially their Loveless album. This album was a headache and somewhat annoying at first, but slowly over time became a "classic" and yielded hours of sonic bliss.
Music you didn't like at first... but now?
Sometimes, there's a sense of discovery or a kind of magic that doesn't sink in right away. Have you heard something that you changed your mind about and wondered why you didn't get it sooner? What artists/recordings have you experienced this with?