metallica- listen to master of puppets, if you don't like it then you will never like metal.
megadeth- awesome speed metal, kind of like metallica but a bit quirky, more diverse riffs, faster, more technical, but less heavy. shorter songs. try peace sells but who's buying, it's a great album.
iced earth- skip iron maiden and go for these dudes. very epic, theatrical stuff, more along the lines of power/classic metal than thrash. try horror show or something wicked this way comes or better yet, get the alive in athens 3 disc live set. they are truly an incredible live band, tight playing and stage presense out the bunghole.
mercyful fate- early 80's black metal, great ski music. try their first two albums. this band will test your attention span, but will definately pay off in the end. there's some surprisingly melodic instrumental passages and absolutely cacophonous riffs everywhere.
motorhead- a faster, rawer version of ac/dc. try ace of spades or overkill.