I like watching shows that help my kids ask lots of questions about growing up, sex, drugs, etc.
Glee has actually been REALLY good at starting some amazing conversations; bullying, sexual activity, and following your dreams have all come up from watching this show.
Secret Life of the American Teenager has also started some conversations - but my son is like "Geez, what's with all the drama?" LOL!!!
Then there are shows that aren't necessarily geared toward teens, like "V." Though it's a Sci-Fi show, my kids have so many questions about how our actions effect others! It's awesome, especially since I love that show LOL!