I can certainly see a point in IGYEIH being on the survey instead of some of the ones I included - it definitely would have if I could put more than 10 options. There are moments on IGYEIH that feel like some of the heaviest stuff the band has done (mostly intro and outro), but I though as a whole the verses and chorus were leaning a bit too much on the melodic side for inclusion. But in the end it's very subjective.
IGYEIH has the most aggressive Emily vocals I'd say. Even in the verses she sounds angry as hell. I suppose the guitar in the choruses is just power chords, rather than a "heavy riff", but to me the slower tempo of the song makes it feel heavier than Casualty, which I'd consider one of the heaviest for sure but I don't think that "frantic" and "heavy" are synonymous here.
There's no universal definition of what makes something "heavy." Most people interpret heavy based on different elements that they find "oppressive" or evoke other similar emotions. So it varies wildly based on what production values incite those ideas for you.