To me it was the If statement, I'm psyched up, since then I believed that computers are very intelligent, or I can at least make it appear intelligent because of it.
When I first started learning programming with QBASIC the whole idea of flow control using if statements and loops was great. I think it was just a few days after I learned about the if statement that I built my first "Choose your own adventure" game. Looking back I know it must have been horribly inefficient and massive in terms of lines of code, but the fact that I could branch the story using nothing but if statements was wonderful.