Anything by Incantation, but I'll go with Ibex Moon.

Death - Left To Die

This fucker set the benchmark for all the other fuckers.

Well that's a very tough call, but the first one that sprang instantly to mind is 'Hammer smashed face', and I'm not even a huge Cannibal fan. I do love that album though.
Death 'Evil dead' and Bolt Thrower 'Anti tank (dead armour)' are also in my top three.

Y'know what? I revocate my previous post after actually listening to my sketchy picks.
THIS is the most truthful answer to OP's question:

Deicide - Satan spawn, the Caco-Daemon

I could go on and "defend" it, but the song tells this tale way more proficiently.

Amon/Deicide - "Lunatic of God's Creation", "Crucifixation" or "Dead by Dawn", preferably from the Sacrificial demo which is possibly the best death metal record ever made.

I still prefer Amon's '87 demo - so monumentally evil you won't notice it's only about 10 minutes long.

I admit the 1987 Feasting the Beast demo is a monument of insane bestiality and savagery. I often wonder why this record gets so little attention. Released the same year, Scream Bloody Gore sounds like funny thrash compared to it.

In the Grip of Winter. Doomy riffs, true horror atmosphere and vocals so sick that they made me laugh. (Laughter is the best therapy.) When I heard it on the radio, Autopsy came to be my favorite DM band. And they still are.

I agree that the Amon incarnation of Deicide contains some of the most vicious, evil, blood curdling death metal ever put to tape. I love that unheralded little orange recording by Deicide-as-Amon that seems to be overlooked by fucking EVERYONE!

That version of Dead by Dawn is fucking unreal.

Would you say Glen Benton used some kind of voice harmonizer or was it 100% his vocal ability? Anybody even knows?

Bolzer - Entranced by the Wolfshook

Yes.

maybe one of the greatest metal songs of the 21st century

Not sure but it sounds to me like they used a guitar pedal for the voice, like a Metal Zone or something, and pushed it to full saturation. In any case, the result sounds insane!

Normal death metal vocalists use (more or less) effects on their voice, so you can easily imagine effects were used to reach such bestial and inhuman vocals!

You guys don't miss. Another banger, and off of their best album too!

That's it, Vader - "Xeper" gets a vote from me.

Leprosy

The one that sings something about violence

He's always sounded to me like there's some pitch-shifting going on (often two vocals tracks with one pitched up and one pitched down) so my guess would be yes.

I think you're not talking about the same thing. Glenn Benton's trademark is indeed doubling his growls with black metal-like shrieks on some parts. But we were talking about the very particular distorted effects he uses on the 1987 demo that has never been heard anywhere else...

I'm going with Death - Pull the Plug

Honorable mentions:
Death - Open Casket, Lack of Comprehension
Carcass - Corporal Jigsore Quandary, Heartwork
Deicide - Sacrificial Suicide, Dead but Dreaming

Oh

Not the most iconic, but I think the greatest that I’ve heard is probably Immolation’s Close To A World Below.
Other worthy greatest death metal songs exist, many which have already been mentioned here.

This whole album.

Yeah that pig squealing bullshit is definitely as good as death metal gets. "Death Metal" at its absolute nadir.

This song has always basically been the epitome for me.

Nile - Masturbating the War God

Oooo that's a good one, there's so many good riffs packed into that one, there's the groovy riff towards the beginning and then that riff with all the fuckin fast harmonics in the pre-chorus.

To me, nothing tops this masterpiece. Epic isn't something I usually associate with DM but this one goes above and beyond in that department, while sacrificing none of the heaviness. Often long songs can feel disjointed, this one really earns its length. Basically the perfect sendoff to 90s death metal (not that it ever truly died).

I have a few choices.

Cryptopsy - Crown of Horns (could have went with Phobophile or Slit Your Guts, but the sample at the beginning of Crown of Horns lends the track such a malignant atmosphere).

Morbid Angel - God of Emptiness

Atheist - Mother Man

Deicide - "Lunatic of God's Creation"

That insane scream Chuck does at the start over that killer riff is wicked and when he utters "No more tomorrow this is your last day... On this fuckin Earth!", fuck yeah! And then that final scream just fuckin nails it for me, some of metal's greatest moments.
Then there's the mighty Pull The Plug and Open Casket...one of the greatest albums ever.

If you don't answer:

Hammer Smashed Face
or
Immortal Rites

You are wrong.

Chapel of Ghouls all the way. Y'all can't change my mind.

w/ regards to Immortal Rites, while it's grown on me, it goes from one of the coolest fucking opening riff to an incredibly boring bridge and boring next few verses and it takes a bit before it gets good again with that horror movie-esque melody that starts around the middle.

Hammer-Smashed Face is fun for like the first minute but after a bit it just feels all over the place, like they just smashed a bunch of riffs together without caring about cohesion too much.

Chronic Infection - Pestilence, the thrashing riffs, the chaotic solos, the relentless beat of the drums, and... Martin Van Drunen delivering the horror.

In a close second, Ridden With Disease - Autopsy; and the third place for Open Casket - Death.

Infected by Obituary fucking devastates and the structure is amazing

This track might have it all:

It's really hard to point out. Some of the tracks already mentioned here are on my personal list as well.
But i'll come up with one that wasn't mentioned and is on my top list: Gallery of Suicide, by Cannibal Corpse. It might be the single most haunting tune I've ever listened to, at least on death metal.

Morbid Angel - Fall From Grace & Rapture

I heard both of these songs when they were brand new. Morbid Angel were one of the earliest death metal bands yet they managed to evolve death metal into the next century at the same time.

Death - Flattening Of Emotions

Also heard this song when it was brand new. When the record started and I heard the Tom/double bass build ups, into those jazzy parts, I knew Death had become a different beast. Once the songs picked up into full force I was astonished. As a drummer myself, this album alerted me that I would need to step up my own skillls to stay relevant.

Unleashed - Dead Forever

Unleashed to my ears made some of the catchiest death metal in the 90’s. If I was to introduce death metal to someone who had never heard it before, I would use this song as the perfect example of what death metal is.

For me definitely Phobophile. Complete masterpiece of momentum and tension in songwriting; there's not a misplaced note on the whole thing. And the way everything fits together - the opening riff is one of the best riffs of all time but in large part because of how flo mirrors the "bottoming out" part of it with his tom fill and how Worm's vocals go perfectly with it. Best death metal song on the best death metal album imo.

HM:

Vehemence - We Are All Dying (probably the one that most challenges Phobophile for me)
Deeds of Flesh - Reduced to Ashes
Suffocation - Thrones of Blood
Cryptopsy - Crown of Horns
Cryptopsy - Open Face Surgery (I know, there's a pattern here)
Brutal Truth - Dead Smart
Amorphis - Drowned Maid
Bolt Thrower - World Eater

Yeah man, this would be my pick. I’m not the biggest death metal guy in the world and a lot of the OSDM in this thread sounds dated to me, but Slit Your Guts (and really all of NSV) still hold up among the heaviest shit I’ve ever heard. I didn’t know music could get that heavy when I first heard it as a young teen. The breakdown/slam/whatever the fuck you want to call it in this song is untouched brutality - the quintessential DM song for my tastes and personal metal history.