Theres so many things out there I could go on and on and on forever and this is a VERY tough call ... but I'm gonna say
"Bloodbath - The Soul Collector" is the heaviest death metal track of all time.
"Abigor - Satan's Galaxy" is the heaviest black metal track of all time.
Below is just rambling and a few other suggestions and random info ....
Abigor ... My favorite Harmonic Black Metal band ... Extremely fast and insane. Be sure to listen to the Satanized and Apocalypse albums. They are both Fing awesome.
Bloodbath ... There are so many amazing death/grind bands ... but this one really stands out. The Lineup is absolutely incredible.
VOCALS: Mikael Akerfeldt (of Opeth, he also was on a couple Katatonia Albums, and Edge of Sanity Crimson),
DRUMS: Dan Swano (he is one of the greatest metal drummers of all time. If you listen to metal, you should know his name. He plays or has played for Nightengale, his own solo project, Unicorn, Edge Of Sanity, Infestdead, Odyssey and, Pan-Thy-Monium)
BASS: Jonas Renkse (of Katatonia & October Tide)
GUITAR: Anders Nyström (of Katatonia, Diabolical Masquerade, & Bewitched)
Be sure to listen to the Resurection through Carnage cd ... IMO it is the greatest death/grind CD ever. The track tittled "The Soul Collector" is most extreme.
Also ... just a few more memmorable tracks that are pretty heavy (and also well played)
Broken Hope - I am God
Isacaarum - pretty much anything by them ... Their C*** Hakners album probably has the heaviest tracks ... a good example is "Isamahaparinibbanasuttam"
Nifelheim - Unholy Death
Naglfar - The entire Diabolical CD
Nazgul - In Summis Montibus
Anything by Pigsty
Enslaved has some pretty insane stuff out
Electric Wizzard - Anything on the "Come my Fanatics" cd. They are anything but fast, however, they play an extremely extremely heavy slow stoner/doom style. Its definatly worth a listen if you are into that kind of thing.
PS ... I am only considering tracks where the band members clearly know how to play and where the music is extremely pleasant to listen to as well.