There are a few, in no particular order:
- Hyperion (series), by Dan Simmons
- The Melancholy of Haruhi Suzumiya (series), by Nagaru Tanigawa
- Kim Stanley Robinson's Mars trilogy.
- The Animorphs series by K. A. Applegate (+ an army of ghostwriters)
- Eureka Seven
- Durarara!! (series) by Ryohgo Narita
- Shinsekai Yori
- Katawa Shoujo
- The Expanse (series) by "James S.A. Corey"

I have to admit I'm not good at expressing why, but all of those have had a significant impact on my outlook, or at least made me cry.