In reply to DaveHK: Second what someone said about just getting texts instead of introductions, we have wikipedia and the internet now which limits the value of "introductions to..." to a large extent.
Ethics and 20th C existentialism are the most accessible to that age imo.
Definitely go with The Stranger/Outsider if you're getting a Camus book, it's short and really readable, the plague slightly less so. Whoever said Myth of Sisyphus was approachable must have been a very bright 11-18 year old!
The big guns you asked about, Descartes would obviously be the meditations but I'd be inclined not to go down that road. It was my first taste of philosophy and while the content is fundamental it's got to be the worst possible intro imo.
Again with Hume, is a teenager really going to stick it, he's very formal and the style is dated. I'm not slagging him but I wouldn't have enjoyed reading it before I went to uni.
If you want something greek maybe a collection of greek philosophy might be more useful than plunging someone into a complete work.
I'd fill the shelves with 20th century existentialism because as a teenager, that's what I'd have enjoyed most. Sartre, Camus, Celine, Nietche, Kafka, Kierkegaard.
Also cover ethics because it's easy to get into and always gets people debating. It's not abstract, it's not complicated, it's not dated, but it sparks an interest and that's what you want I guess. You start by talking about abortion and then before you know it you've skipped past consequentialism and are in the depths of a meta-ethical crisis!
Do get something by mill though, utilitarianism or on liberty.
The penguin great ideas is a good collection of short works as well.