I wrote my list in the shadow of fear of a certain pompousness, but the specification of novels, plus my assumption that we're thinking of the impact of their entirety, imposes constraints.
Not many novels accessible to pre-adolescents are going to be able to say something lasting about the human condition. The books on science in general and cosmology in particular that I devoured before high school influenced my path far more than I think any fiction could. As did _The Chomsky Reader_ later alter how I view the world. Or how, say, _A Brief History of Neoliberalism_ consolidated and fleshed out what I already knew. Effective novels are more subtle than effective nonfiction.
A reoccuring problem is how examples of sci fi often fail as a whole work, instead of nuggets of ideas and imagination wrapped in near afterthoughts of narrative. The nuggets have impact, but the works don't. _Ringworld_ comes to mind, as does just about anything by Neal Stephenson. His _Diamond Age_ muses on the how a culture's values can influence its viability, considers learning and childhood development, and ponders the meaning of "hypocrisy" all in a package, with all his usual insightful observations and lame ending, that manages to crudely objectify the entire country of China. Impact, sure, but a mixed one.
-- Andy