Most Helpful Girl
It's a hard tie between The Time Machine by H. G. Wells and Solaris by Stanisław Lem.
The former is your typical science fiction, loading equally heavy on the science and the fiction. The latter is both science fiction and philosophy, loading VERY heavy on the philosophy--in particular, it's kind of existentialist and absurdist, and will probably leave you with a bit of a bleak, "what the hell just happened" feeling after you read it.
Most Helpful Guy
Almost anything by Orson Scott card or the epic series by Conor kostick