Definitely Metallica! I had heard of but did not listen to Metallica before Guitar Hero: Metallica came out so I didn't really have an interest in the game, but I rented it because I love Guitar Hero and ended up really liking their music.
Avenged Sevenfold! I thought all modern rock completely sucked until I heard them. And for some reason Paramore, "Thats What You Get" from RB and "Misery Business" from GHWT caught my ear. I like her voice.
Not a band, but a whole genre. I used to hate metal music, mostly because it was mostly what my dad listened to, and that fact alone made me dislike it. After going through Cowboys from hell and bark at the moon with the first GH, then through Hangar 18 on GH2, I realized the error of my ways... I was a closet headbanger
I love rise against thanks to the bonus song Prayer of the Refugee in Guitar Hero 3.
As soon as i got that song thats all i played. Now i own most of their cds and they are definitely my favorite band.