These are my favorites:
1. Skins (All generations, they're all so different but I still love all of them)
Now these are mostly just from E4 and are kind of just high school shows but I still absolutely love them. Skins for its totally irreverent way they look at teens and the glamorous look of trashy teenage lifestyle. Misfits is like Heroes but all teenage delinquents with super powers and a lot more swearing. And Inbetweeners is the British version of any normal guy's high school experience.
Unfortunately Inbetweeners has ended but I am still WAITING IMPATIENTLY for the other shows to start again.
If any of you have any more shows to recommend I am totally open to it. British or otherwise.
I might later do a post on great American shows... but I'll have to think carefully about that.
The show Skins' writing team has an average age of 21 and includes several "teenage consultants."