As a massive Heinlein fan I’ll say Starship Troopers because I think that’s a fascinating discussion about what you owe your country or the price of citizenship. Since Heinlein is my favorite author I’ll pick another one as well.. A Scanner Darkly by Philip K Dick. I know he had drug issues and this book is an interesting look at the cycle of drugs and addiction and the ending was tragic but also very realistic. I like that the movie included his tribute at the end.