Man, I saw this thread and had to chime in because 2015 feels like a weirdly underrated year now. The one I keep pushing on people is 'A Little Life' by Hanya Yanagihara. It blew my reading group apart—half of us thought it was this profound examination of trauma and friendship, the other half called it trauma porn. I'm in the first camp. It completely rewired my brain about how far a character study can go, even if it's brutal.
Beyond that, I remember 'The Story of My Teeth' by Valeria Luiselli being this playful, experimental thing that was nothing like 'A Little Life'. It was smart and funny, about a auctioneer with fake teeth, and it played with form in a way that felt fresh without being pretentious. 'The Sellout' by Paul Beatty won the Booker the next year, but it came out in 2015. That satire is so sharp it could cut glass; it's hilarious and vicious about race in America.
If you want something quieter and more atmospheric, 'The Mare' by Mary Gaitskill is stunning. It's about a troubled girl and the woman who fosters her, connected by a horse. Gaitskill's prose is just unbelievably precise, finding emotion in the smallest gestures. And don't sleep on 'The Tsar of Love and Techno' by Anthony Marra. It's linked stories set in Chechnya and Russia, and the way he weaves the narratives together with such humanity amidst the bleakness is a real masterstroke. 2015 had range.