Basically, fans will argue any full-length from 1989-1999 to be Reznor's Magnum Opus.
Year Zero, a grim Concept Album composed muchly on Reznor's laptop and featuring arguably some of his most visceral work in years. It's by far his most critically acclaimed work of the 2000s.
Hesitation Marks. It's proof that an artist known for writing angst anthems can indeed come back with a mostly upbeat and positive-sounding album and not lose any of his spark or integrity. But if we had to point out specific examples:
"Copy of A". The very definition of simple yet effective.
"All Time Low". Trent getting in touch with his sexy side and it's hard not to love. And the outro... that outro...