Star Trek (TOS)
I grew up watching Star Trek's original series as a young boy in the 1970s and it will always have a special place in my heart. There's no doubt watching the syndicated re-runs everyday after school as a 10-year-old helped shape my world view, which is that of a humanist purveyor of whatever-floats-your-boat acceptance.
Space hippies? I can see their point. Co-exist with a rock-eating monster? Why, I'd love to. Spare the life of lizard-man trying to kill you? Without question. I adore TOS and marvel at the way it has stood the test of time.
Any number of a dozen classic sci-fi episodes could have topped my list, but ultimately my No. 1 choice defines me as a person more than any other episode of Star Trek. (Hint: I love animals.)