Depends entirely on what they've listened to in the past. If they were classical fans, I'd play them five prog rock albums. If they listened to Jazz and Blues I'd play them Steely Dan and Derek Trucks. If they listened to pop I'd play them some Indie Rock. The thing about introducing somebody to any genre, especially rock which probably has more subgenres and variety within the genre than any other one, is that if it's too different from what they already know, they probably won't like it. You can't just play them the classic albums by all the godbands and expect a jazz fan to just automatically love Led Zeppelin. Give them what sounds closest to what they like and slowly expand until they're open-minded enough for other things that don't sound like the genres they first listened to.
Led Zepplin - Led Zepplin IV
Rolling Stones - Sticky Fingers
AC/DC - Back In Black
Jimi Hendrix - Band Of Gypsies
The Allman Brothers Band - Live at The Filmore 1969
Whoever said "A Little Bit Longer" has a little bit longer before I punch them in the face. That's just about the furthest possible thing from ROCK. On top of that... they're a less-educated version of Titan without proper capitalization, grammar, and punctuation. For cryin' out loud.
Anyways, it depends on the kind of rock. But in general, I'd use (1) Nirvana - Nevermind (2) Pearl Jam - Ten (3) The Beatles - Sgt. Pepper's Lonely Hearts Club Band (4) Black Sabbath - We Sold Our Souls for Rock N' Roll (5) Guttermouth - Musical Monkey
I'd use #1 and #2 because they brought a revival to Alternative and popularized Grunge. Plus, #2 is my favorite album and really recommended. I'd use #3 because that's usually the first album that comes up when people say ROCK. It's the real classic stuff and gives someone a look at what rock was in the 60s... and then rock in the 90s. Plus, it's a must for classic rock fans. I'd use #4 because... well, that's pretty self-explanatory. Awesome band, awesome album, and a taste of a much heavier version of rock: Heavy Metal, the 70s and 80s British Heavy Metal, the way it should be done. I'd use #5 to show that rock can be really sarcastic and ridiculous, too, and to show that rock is all about having fun and being an individual through your music, you know?
So really, I'd use them to help the person realize the different types of rock.
That way, they could find one that they took a particular liking to.
Led Zeppelin 4 - Led Zeppelin
Exile on Main St. - the Rolling Stones
Toys in the Attic and Rocks - Aerosmith
Are You Experienced - the Jimi Hendrix Exp.
Live at Leeds - the Who