There are more technically heavy metal options in New York. You can do metal karaoke at Arlene's Grocery and Continental. Sweet Paradise and AdLib are hosting a heavy metal and hardcore night, while Duff's in Brooklyn is “metal themed” with a mostly metal juke box that was adequate.
But if you want to go to the dirtiest most disgusting dive bar in the city, where you feel kind of scared even when the sun is out, where the man next to you is either passed out or dead, and the man next to him is screaming, nothing holds a candle in flat out sordid decay than Mars Bar. I can't even begin to imagine the amount of urine, feces, and vomit the floors and walls have seen.