Infinite Jest by David Foster Wallace has been the most impactful, but due to the length of the book it's hard for others to compete on the impact-per-page metric. I wrote about it in detail here https://speak.sh/posts/infinite-jest

The Unbearable Lightness of Being by Milan Kundera showed me another perspective of love I had never considered. Gave me insight into vulnerability.

Flowers for Algernon by Daniel Keyes showed me how by looking through the world via a lens of intellect, you can often miss the point.

The Bus Driver Who Wanted To Be God by Etgar Keret a collection of some of my favourite short stories. Highly recommend.