I'm pretty indiscriminate in my reading habits, as long as it has some speculative aspect. As my last book post shows, even that's not necessary for me to read a book and get something out of it. Still, books that are fun, quick, and have compelling characters, plots, and focus on intriguing issues are the ones that I seem to like best and stick with me the longest. Frequently, those books are YA.
I don't think I could list all my favorite books without a hefty dose of YA. "The Giver" is one of those books that stuck with me for years as I'd think about what defines evil and good and who has a right to life. More recently, "The Hunger Games" and the rest of the trilogy did a fantastic job of showing the horrors of war. Dan Well's John Cleaver trilogy was an awesome exploration of what it means to be human or less than human. The ending is still one of my favorites--Wells puts you through an emotional wringer and still manages to pull off an amazingly hopeful ending.
If you love YA you should head over to Beth Revis' blog and check out her ***50 BOOK*** giveaway. It's colossal. Also, fantastic selection of books.