Granted I haven't heard a hole lot of things from each decade before the 80s, but bare with me.
50s: Miles Davis - Kind of Blue
60s: King Crimson - In the Court of the Crimson King
70s: Joy Division - Unknown Pleasures
80s: Talking Heads - Remain in Light
90s: Radiohead - OK Computer
00s: Converge - Jane Doe
10s(Very early, but so far, my pick would be...): Swans - The Seer(Though Deftones' Koi No Yokan and GY!BE's Allelluyah! Don't Bend! Ascend! may take it down after a few more listens)