The most obvious one, Stairway to Heaven. Biased because it's my favorite of theirs, but it's popular for a reason, and I think anyone who listens to it would be pretty well setup for the rest of their music. Honestly, I think any song in their Best Of would be great for someone unfamiliar with them, like I said, their most famous work is popular for a reason, it's all amazing. If you like that stuff, you'll love everything else just as much.