One of my favorite books when I was a kid was The Andromeda Strain. I read it when I was probably around 10 or so, and I can safely say that this was the book that made me want to be a scientist. The concept was so scary, and the solution to the "mystery" of what the alien superbug was and how it worked was so logical that I found the story incredibly fascinating.
Strangely, I had no idea a movie based on the book had been made. I just watched it, and despite having been made in 1971 it still holds up incredibly well. It's very true to the book, and the directing makes the whole affair feel unsettling and eerie. I was literally biting my nails near the end of it - and I already knew what was going to happen from reading the book! Most of all, I loved that in the movie, the character of Dr. Leavitt is a woman (opposite of the book.) She's smart, she's tough, she has attitude - and all the men accept her as their scientific equal.
Anyway, I'd say this one is a must-see for any sci-fi fan.