Before explaining, I just want to say "Friends of Hell" by Witchfinder General has to be mine as a whole, but "The Prince" from Diamond Head and "Motorbreath" from Metallica (throw in "Bottoms Up" from Acid to be safe) fills it out overall.
What I mean by this is a song that, to you, is that well done to the point every second is a pure eargasm. Everything has been done right. It's a song you could put on repeat all day long and never get tired of it. It has just the right speed, just the right riff, just the right heaviness, just the right lyrics (if any), just the right presentation, just the right everything. These songs have things you prefer to hear in metal songs.
My own preference for these things-
No hanging riffs. Candlemass, Saint Vitus, Electric Wizard, I love you guys, but I don't remember Black Sabbath hanging any riffs. (i.e. there's a note in the riff that sounds just slightly off from the rest and almost makes the whole riff sound incomplete or even dumb. Candlemass's "Solitude" has a hanging riff. These things piss me off.) Songs that are rifftastic and do them right, I rate high. Vein of "Vinum Sabbathi".
No awkward drum sections. Mob Rules, by Black Sabbath, is a great hard rock piece, but I just feel that the drums were butchered in that one. That's not to say bluesy-time signatures don't jive with me. Anything but.
Great speed or slowness. For each and every tempo, I have a desired sweet point. My absolute favorite is what Metallica pulled off in Motorbreath and Whiplash.
Keeping speed. I dub this 'thrash rocking', as a song where the whole thing doesn't slow down for too long (too long may mean 1 second). Highway Star ala Deep Purple, So It Goes by Nebula, and The Prince from Diamond Head are great examples.
You say "Justin Bieber", I say... OK. So?
92% of teens have cleanly divided themselves according to genres. If you're part of the 8% that doesn't give a shit why others listen to their music, then I don't care. Just enjoy the damn music.