Some of my favorites:
When the distorted guitars enter in PMU
LTL, the second verse
The Mirror, especially the outro
PS's second half
BtL, right before the instrumental section. This may not be all that heavy, but it is one of their most aggressive riffs.
TGP in its entirety, along with the rest of the 12SS
All of Train of Thought
TDEN, probably my favorite heavy song of theirs
BITS, the moment the guitars enter is one of my favorite DT moments
BTV, I love this one. One of their best concise songs