SF for beginners

Which genre books would you recommend to your bookworm friends who haven't yet discovered the joys of science fiction and fantasy? In this event we are looking for precisely those books that can be described as "gateway drugs" into the world of SF! You are thus invited to briefly present one or more genre books (science fiction, fantasy, horror, etc) that you reckon have the potential to convince readers of mainstream fiction to give SF a more serious look.

Afterwards we will have a casual and friendly chat about the books, about the subject of beginner-friendly literature, and wherever else the conversation takes us!

You are of course welcome to bring the books you want to discuss, but it's also fine if you don't.