Mainly Metallica and the Moody Blues. I used to think both of them were pretty crappy, and now I love them. More recently, Electric Wizard sounded pretty bad when I first heard them, and I ended up really getting into them (although I still think their songwriting is thoroughly mediocre).
I know there are a lot of styles of music which I used to dislike years ago, and now enjoy - things like jazz-rock, blues-rock, oldschool punk, and various mainstream rock bands from the '60s and '70s. I couldn't really name a lot of specific bands I'd heard back then that I hated, but most any band of those styles would have evoked nausea in me had I listened to them years ago.