Regarding The Chasm, I think that "Procession to the Infraworld" (1999) is that bands only truly excellent work. By comparison, all subsequent works are somewhat repetitive and burdened by the 80s thrash and NWOBHM nostalgia. "Procession to the Infraworld", on the other-hand, is a masterpiece and certainly one of the best albums from between 1999-2009.
I aso think that Absu - Tara is an absolute must. That album is like the sum of all thrash knowledge: seamless integration of that style into an epic, narrative format.
Out of all the bands/albums that have been mentioned, those are the only two that I would absolutely insist on, especially if this list is like a top ten or something, instead of just a catalogue of albums that were generally good/tolerable/interesting.