I have to pick 2 songs for this 5 hugely influential bands I really love (in no particular order):
Judas Priest: Painkiller/ Hell Bent For Leather
Devin Townsend: Namaste/ Almost Again
Rosetta: TMA 3/ Red In Tooth And Claw
Paradise Lost: Encantment/ One Second
Iron Maiden: Aces High/ Rime Of The Ancient Mariner
(Have to drop Sabbath, Cynic and Cult Of Luna but honorable mention nonetheless)
... but if I had to show someone totally new to metal what the whole thing was about it would be a compilation with these 20 songs:
1. Black Sabbath - Black Sabbath: "See man? this is how it all began..." (Heavy, Doom, Blues)
2. Judas Priest - Dissident Aggressor: "... then these guys came along with an aditional guitar and a zeppelinesque vocal style and defined it." (Heavy/ Power Metal)
3. Rush - Cygnus X-1 Book II Hemispheres: "Remember what prog bands were playing back in the turn of the 60's to the 70's? Apply a bit of sabbath to it" (Prog Metal)
4. Motorhead - Overkill: "Dirtier, faster and more aggressive than anything ever made" (Heavy Metal, Punk Metal)
5. Iron Maiden - Phantom Of The Opera: "Speaking of dual guitars... this is the biggest band in the genre. Quite unique sound signature" (NWOBHM, Maiden Sound)
6- Metallica - Fight Fire With Fire (or Megadeth - My Last Words): "Based on the NWOBHM with a much faster tempo and aggressive guitars" (Speed/Thrash Metal)
7- Celtic Frost - Procreation Of The Wicked: "This guys came from out of nowhere with the darkest and most evil sound one could imagine. Heavy and eerie as hell" (Dark, proto Black Metal)
8- Bathory - Total Destruction: "Northern Mythology or Satanism themed, croaked vocals, and also heavy, dark and fast as hell" (Black Metal)
9. Slayer - Angel Of Death: "Take speed/thrash and redefine the notion of the words heavy and fast" (Thrash Metal, proto Death Metal)
10. Suicidal Tendencies - Subliminal (or S.O.D. - Milk): "By this time some guys started mixing Thrash with Hard Core - Punk" (Crossover/ Thrash Core)
11. Death - Leprosy "Then things started getting a bit heavier, faster with growling vocals..." (Death Metal)
12. Terrorizer - After World Obliteration (or Napalm Death - Instinct Of Survival): "... ok, here it gets way heavier with blast beats all over and a punk groove" (Grind Core)
13. Godflesh - Like Rats: "Samplers, Drum Machines and metal with a factory like ambiance" (Industrial Metal)
14. Pantera - Cowboys From Hell: "a bit similar to thrash but at a slower tempo, full of harmonics, slides and pulls" (Groove Metal)
15- Cathedral - Frozen Rapture: "Slow tempo as slow can get, with crunchy guitars and growling vocals. No happy tunes here." (Doom Death)
16. Paradise Lost - Christendom "Get a couple of metal infused riffs, Female Vocals, keyboards, a Goth rock influenced lead guitar and of you go!" (Gothic Metal)
17. Clawfinger - The Truth: "Hey! Who would say Metal and Rap would ever mingle?" (Rap Metal)
18- Korn - ADIDAS :"Downtune your guitars, alternate the vocals between screaming and moanings and if it's the late 1990's you would get rich easily
" (Nu Metal... yuck!)
19- Neurosis - To Crawl Under One's Skin: "Weird, dissonant and depressive long compositions featuring sludgy guitars and walls of sound" (Post Metal/ Sludge)
20- Meshuggah - Future Breed Machine: "Complex structures, thrashy guitar riffs in bursts with intricate tempos and a slight prog feeling underneath" (Djent/Extreme Prog Metal/ ... whatever you wanna call it)