I know most people rave on about his Animorph series, but what really did it for me was GONE and BZRK. Both of those series made me go from being a light reader to becoming a full-on bookaholic. This is the most wonderfully, fantastically messed up author I know, and I love it. No other YA author can make severed limbs, organs and evil supervillains be so entertaining.
The concept of the stories are refreshing, completely different to anything I'd read until then and, honestly, to anything I've read since, and that is why I've kept this author as one of my all-time favourites. If you enjoy books that aren't afraid to go way past where YA fiction usually goes, this is the author for you. If you want the weirdest, coolest concepts in current YA fiction, pick up one of his books.