I'm surprised by how much the Hunger Games was mentioned. I do recommend it. The problem with that book, though, is that if you read it, you will read the second as well. While Catching Fire had decent action, that was completely swamped by the romantic crap. If you're like my friends, content to sit around moaning about which boy the female protagonist should choose, do read it. (Oh, lord, how I hate them. "Like, I'm so mad! I can't believe she chose him! OMG!")
I enjoyed Lord of the Flies, although it was creepy as fuck.
One of my favorite books is Un Lun Dun, by China Mieville. I thought it was clever, cute, and not too childish, although the environmental messages are less than subtle.
I'm currently reading Watership Down. When my friend told me to read it, she only mentioned the title, and I thought it would be about a sinking ship. It was about bunny rabbits. However, it is an interesting and emotional read. And hey, now I know what the Warriors series was ripped off from.