I always consider GnR, Motley Crue, and the rest of the 80s hair bands, and most of the foundational 70s stuff like Zeppelin and Thin Lizzy, "hard rock" while I reserved "heavy metal" for bands like Slayer, Iron Maiden, Anthrax (not very good IMHO), Pantera, and Megadeth. Then there'st the so-called "black metal" or "death metal" bands like Sepultura, Canibal Corpse, Napalm Death, etc. Kind of surprised nobody mentioned any of them. Had a friend in college who used to explain the musical structure behind death metal music... It's loud, aggressive, and very repetetive.
Anyway, Metallica is by far the best of this genre that I know of, and I think Iron Maiden, esp. their first album, is also quite good. Some Megadeth stuff, esp. Rust in Peace, is also good, though Dave Mustaine's squeeky voice and shallow political ravings are annoying as heck.
Did I mention that this is a really cool site?