Veronica Mars (2004–2007)
I loved, loved, loved this TV show. The best thing about it is the character of Veronica. I think she's 17 in the beginning episodes of the show (maybe even 16), and she's one of the smartest TV characters I've ever seen. She's not just private eye smart (she's the daughter of the local P.I./former sheriff and does her own sleuthing on the side); she's also relationship smart. And when was the last time you saw a TV show character who wasn't a total idiot about relationships? I finally stopped watching Grey's Anatomy because I couldn't stand the retardation of pretty much every character. (oops! I accidentally slept with so-and-so! i must do everything possible to keep what's-her-name from finding out! that is the best thing for our relationship!) There's a reason I don't watch daytime soaps. (And mostly that reason is that the plots proceed at a snail's pace, but also, it's the stupid relationship stuff.)
I'm not saying that Veronica never makes a mistake or does something stupid, but when she does, she takes steps to correct it. She's honest and seems to actually have some principles. I know Buffy is another strong, smart, young female lead, but Veronica is even better: she battles the bad guys (and they're just regular, non-supernatural bad guys, which is nice), she does well in school, she helps out the unpopular kids, she gets along with her dad (of course, her dad is another excellent character). She's not perfect, but she's a great person.
The story is good, too. The first season has one overarching mystery throughout, with one-episode P.I. jobs along the way. The second and third seasons have shorter, multiple-episode mysteries. There's good writing and decent acting. I fully intend to make Jon watch it with me someday.
The disclaimer: As the parent of teenagers and a former naive teenager myself (at least to some degree), I didn't like all the drinking and drugs and mean people and sex in the show, but I know from other, more experienced people that it all really happens, at least for some kids. Just thought I'd warn anyone who's interested: there is plenty of bad stuff.