Okay, fine! I'll have the balls to step up to the plate for this one.
This is my poorly attemptedly semi-objective pseudo-chronological "hit all points stylewise possible, albeit with a slant towards the recent and taken from my own personal biases, minus a few exceptions (*cough*tallica)" list.
1. War Pigs - Black Sabbath
2. Angel of Death - Slayer
3. Master of Puppets - Metallica
4. Metropolis Pt. 1 - Dream Theater
5. Twilight - Edge of Sanity
6. Preludium/To Mega Therion - Therion
7. Advent - Opeth
8. The Loss and Curse of Reverence - Emperor
9. Artifacts of the Black Rain - In Flames
10. Dead Inside - Arch Enemy
11. The Kinslayer - Nightwish
12. The Enigmatic Spirit - Vintersorg
This is more of a "these are the different ways metal can sound, and an introduction to some of the best artists in metal" than "these are the 12 best metal songs ever" compilation. Otherwise it'd just be 1-9 MAYH, and Advent, BRI, and TBYF.
I felt I had to put Sabbath, Slayer, and Metallica on just to establish history, I think...