1) It's lyrical subject matter is heavy and super controversial even to this day.
Was it more controversial for it's era than Black Sabbath? I wouldn't say so...
When video game executives won't let you put the song on Guitar Hero because of lyrical content even at the insistence of fucking Lars Ulrich, you know you've got something special.
That was over 20 years after it's release. 'Impact' is supposed to be initial, right?
Making bigwig white collar movers-and-shakers uncomfortable because of ugly truths of humanity is what heavy metal is all about.
Perhaps, but that doesn't really fit into the criteria, and it's subjective anyway.
2) Singlehandedly set a new musical standard for thrash metal (much more so than Master of Puppets)
Be careful about using words like "singlehandedly".
I would say it was the Reign In Blood album AS A WHOLE that set a new standard for thrash, not that song in particular.
3) Is probably the only song from the last 25 years (MOP aside, duh) which could be claimed as "the greatest" which could be taken as a serious suggestion.
Maybe, but does that make it greater than Black Sabbath?
Metal has been around much, much longer than 25 years. In the grand scheme of things, I still say Black Sabbath's influence is untouchable within metal overall. Not just because of the possibilities it introduced, but also because you can just HEAR the influence of it in so many metal acts who followed.
With point #1, in regards to the point you made about shocking, scaring, or at least making the audience uncomfortable; look, most of the songs in the top 10 have been assimilated into mainstream culture at some point, but Angel of Death? Not quite.
And this just proves that AoD lacks popularity...what's your point?
Again, even though LARS FUCKING ULRICH, the guy who all the corporate suits look at as their greatest ambassador to metal because he's made them the most money, asked for Angel of Death personally to be on his game, they still denied him. If that's not metal, I don't know what is.
And yet, Black Sabbath was recorded at a time when the airwaves were ruled by psychedelia, pop, and blues rock.
It was a whole other beast.
Impact? Of the three thrash giants that came out in 1986, this one had the least promotion, and it still became legendary. A big part of it was Angel of Death.
What are the big thrash giants of 86? Puppets, Peace Sells and....To Mega Therion?
Popularity? because of it's touchy subject matter, it will never become bigger than any Sabbath, Metallica or Maiden hit, but I'd say it's the most popular "underground" metal song ever.
Black Sabbath had equally touchy subject matter and it became one of their biggest concert favorites. Plus it's praised by a whole slew of metal artists.