How can we measure such things? The Hardcore factor of music must always be rated in seven steps. For this discussion, I will remain focused on metal to narrow the scope of discussion and make it more comprehensible.
Zero would imply there is no real hardcore element involved, however entertaining the artists may be. Metallica, Iron Maiden, and the like would be ascribed to this layer, which is kind of a non-hardcore layer. (Anti-hardcore bands, like Linkin Park or Korn, would be in some Negative Hardcore Dimension where they can wallow in how much they suck and not bother anyone else.)
The higher you go, the more obscure and difficult artists become (therefore you have to be more hardcore to dig it). Beyond the extreme aggression and pulverizing onslaught of SYL, there is depth and cunning. Many people don't see it (indeed, their third album -- which shows this most explicitly -- seems to be regarded as "way less heavy than _City_" and "tame" or "boring" by many fans), but if you had been there to really watch their show, it was a lot more obvious -- especially with the older songs given the extra brutal, raw & bloody sonic treatment of _S.Y.L._.
I think SYL's third album, the one that seems the LEAST liked, is by far the band's PUREST album. It just seems to capture what they are far better than their earlier albums. There is a definite parallel to Naked City's _Absinthe_. It is not my favorite Naked City disc (that would be _Leng Tch'e_), but I would say it purifies the band's disposition so that you can truly uncover its evil nature. _S.Y.L._ is much the same. You stip away all the excesses and you are left with an embodiment of hellfire. And without its disarming illusions, it is far more deadly and dangerous.
You would probably find Meshuggah, Opeth, and Isis on level 4. Ulver and Cynic would stand on the fifth level. Garden Wall and maudlin of the Well are sixth level bands. Low-to-mid hardcore bands would be Dark Tranquillity (level 3), In Flames (level 2), and Nevermore (level 1).
Very rarely can a band reach the seventh level. Perhaps King Crimson belongs there, and perhaps Kayo Dot will prove that they can enter that realm (I have high hopes for them). Tool belongs on the seventh level due to sheer depth. On the one hand, Tool could probably be lumped into level 0 or 1, because they are very popular and accessible. However, Tool's explorations plunge into depths that reach the seventh level, and most people are unable to follow them there. Hence, I think Tool is highly UNDERrated. In fact, if any band can penetrate to a level beyond the seventh, it would be Tool.
Anyway, Strapping Young Lad is a band that MUST be experienced live. When I saw them in Regina, mediocre sound quality kind of put a kink in the show, but if the sound is good (as it was in Calgary last night), you can only be overwhelmed.
The lesson to be learned: Strapping Young Lad is the heaviest band in the universe.