I'll admit my tastes run toward the mid-century stuff, so my list skews in that direction. You can't talk about this without mentioning 'The Grapes of Wrath'. It's monumental, but for some reason, I feel 'East of Eden' has aged a bit better for me—the whole Cain and Abel thing just hits harder on a re-read. It feels less like a historical document and more like a story that keeps unfolding.
For sheer linguistic muscle, 'Lolita' is unavoidable. It's a profoundly uncomfortable book, and Nabokov's command of English as a non-native speaker is frankly showing off. It's a masterpiece I don't exactly love, but I have to respect. Meanwhile, 'The Sound and the Fury' is the one I pretend to have fully understood more than I actually have. The first section is a genuine challenge, but once you get into Benjy's head, the emotional payoff is weirdly immense.
I'd argue 'Beloved' belongs on any 20th-century list, even though it came out in '87. It redefined what a historical novel could do with trauma and memory. And for a wild card, I always throw in 'White Noise' by DeLillo. It captures that late-century paranoia and consumerist haze better than anything else I've read.