Here is what I would do. Introduce them not just to Led Zeppelin's biggest or most popular songs but to their versatility.
1.Pick a hard rock hit. Something like Black Dog, Rock And Roll, Whole Lotta Love or Heartbreaker. If you go with a song like Heartbreaker or Whole Lotta Love use a live version where Jimmy Page really tears it up on guitar such as the Heartbreaker from How The West Was Won or Whole Lotta Love from The Song Remains The Same.
2.Pick an acoustic or mellow song. Something like Going To California, Tangerine, Ten Years Gone or The Rain Song.
3.Pick a unique song. Something that's unlike anything you're likely to hear from anyone else. Something like No Quarter or a live version of Dazed And Confused.
4.Pick a blues song. Something like You Shook Me, The Lemon Song or I Can't Quit You Baby.
5.Stairway To Heaven. It's the most popular song ever for a reason. If you've never heard Stairway you've never heard Zeppelin. If you've never heard Zeppelin you've never really heard real music. If you've never heard real music you've never really lived. If you've never really lived your practically dead. Don't let your friends be dead. Play them Stairway To Heaven.
It's impossible to really expose someone to the greatest band ever with just five songs but if you have to I would do it like outlined above. I've known people who think they hate Zeppelin because all they know is Black Dog and Whole Lotta Love and other of the most popular radio hits today. They had no idea that this band produced songs like The Rain Song, Ten Years Gone, All My Love, Hot Dog, The Lemon Song, Your Time Is Gonna Come and on and on. They just heard a few heavy songs and thought they were just another heavy band. Obviously if you know the people you can find songs they are more likely to enjoy. But with no knowledge here is where I would start with.