Diffie–Hellman. I know that cryptography can get much fancier and more clever, but Diffie–Hellman took a concept my intuition told me was impossible and showed that it's possible in a really simple, elegant way. Learning about it was the first time I realized how beautiful the math behind computer science is. It's also a great insight into just how fundamental the concept of computational complexity is.
So true. Securely exchanging keys is an abstract concept until you see those paint colors mixing!