Have not read much from the 21st but I from I have read I would opt for 1980s: Confederacy of Dunces 1970s: Tinker Tailor Soldier Spy 1960s: Master & the Margarita 1950s: Lolita 1940s: Animal Farm & 1984 1930s: 1920s: Ulysses 1910s: Remembrance of Things Past 1900s: Hound of the Baskervilles
For me personally, 1900's: The House on the Borderland 1910's: ... 1920's: Red Harvest The Great Gatsby 1930's: At The Mountains of Madness Invitation to a Beheading 1940's: ... 1950's: Catcher in the Rye Wise Blood Speak, Memory 1960's: Pale Fire In Watermelon Sugar The Bell Jar H.M.: Solaris In Cold Blood The French Lieutenant's Woman 1970's: Fear and Loathing In Las Vegas Tinker Tailor Soldier Spy H.M. Almost Transparent Blue The Unlimited Dream Company 1980's: The Wasp Factory Blood Meridian Watchmen H.M. Empire of the Sun A Maggot Geek Love 1990's: The Virgin Suicides The Savage Detectives H.M. American Psycho Glamorama The Wind-Up Bird Chronicle