Metallica and Slayer, for all the criticism they receive, y'know, introductory band, old and not good anymore, changed their sound too much, blah blah blah, they're still two of my favorites. They helped me with my rhythm guitar abilities, and served as a good learning tool for beginner solos.
Beyond that, there's tons of bands I enjoy. Too many to list here, actually. The obvious bay area thrash bands are some of my favorites like Testament and Exodus, but I also enjoy the more garage bands like Death Pit, Brugden, Nightkin, Helloween, Halloween, Repulsion, and on and on it goes.