There are a number of contenders, but for me This Dying Soul is the one that stands out.
One of the few of their songs which is unrelentingly and one-dimensionally (in a good way) heavy, from the word go. Straight in with the double-bass and it just builds from there, with the dark, aggressive and percussive vocals, and the riffs, solos and fills just keep getting heavy and crazier right up until the climax of the song, with that insane unison at the end.
TGP is also a contender of course, but even that has more melodic parts than TDS, as it seems that with TDS they really tried to push the heaviness aspect even further (which was the intend behind Train of Thought as a whole, of course).