I'll jump in!!!
BTW- this is really my opinon
I guess we really need to get to the roots:
Black Sabbath- this is really where "metal" started, and is a good place to start.
Judas Priest- Continues in the same vein as Sabbath, but a newer edge to it plus a new musical venture in that there are 2 guitars (sabbath used one)
Iron Maiden- Much more melodic than Priest and Sabbath, using the dual guitars for harmonies (alot of melodic bands quote them as influences)
Metallica- early Metallica (first 3 albums) added punk influences, much faster than earlier bands
Alice In Chains- the metal version of grunge, darker themed music, and even darker melodies
KORN- I will get shot for this by some people, but KORN did bring something to the table, the 7-string guitars they used brought new sounds, and using rap/R&B styling in their sounds
I guess that's were I would start, current bands: (once again, Greg, anytime man)
Bridge gap from mainstream
Stone Sour
Megadeth
Staind- (more hard rock, but bleeds into metal once in a while)
Godsmack
Disturbed
a bit heavier
Killswitch Engage
All That Remains
Trivium
Bullet for My Valentine
In Flames