1. Ender's Game- This is great for kids and adults. It is an amazing and brutal work. The downside of this recommendation is that some people are Orson Scott Cardaphobic. They refuse to recognize him as a person because of his beliefs. Don't recommend this book to those people.
2. The Martian- Engineers will absolutely love it and so will most other people. This book is great for people that have never considered Sci-Fi before. The reason is that it is set in a plausible future with hard science throughout. You might have to tell them it was Sci-Fi after the fact.
3. The Handmaid's Tale- We all know people that think they are too smart for Sci-Fi. This book is smart and bleak. Intellectuals will love it for those reasons. The Road can also be substituted here. Neither of these authors will even admit that the books are Sci-Fi so your too smart for Sci-Fi friends can feel safe in the knowledge that they have been reading speculative fiction.
4. I, Robot- This book is great for people with short attention spans or who don't necessarily want to read an entire book. Each story is independent but pulls you in more and more. I think this was my first introduction to literary Sci-Fi. My mom told me to read it.
5. The Rookie- Know any sports fans? Just start talking to them about football and the future of the sport. Then you can tell them about a football book that takes place in the future with different alien races playing positions suited to their abilities. I bet they will find it interesting to hear about alien wide receivers that can leap 20 feet into the air to make the catch.
Got any other ideas about great introductions into Sci-Fi, let me know in the comments below!