They're not all from 2015. And they're in the order I read them because I don't feel like ranking them.
  1. Americanah by Chimamanda Ngozi Adichie
  2. Shatter Me (series) by Tahereh Mafi
  3. Luckiest Girl Alive by Jessica Knoll
  4. Hausfrau by Jill Alexander Essbaum
  5. Fates and Furies by Lauren Groff
  6. The Panopticon by Jenni Fagan
  7. I'll Give You the Sun by Jandy Nelson
  8. The Wrath and the Dawn by Renee Ahdieh
  9. Lair of Dreams by Libba Bray
  10. Jane Steele by Lyndsay Faye