Animorphs Series by K.A. Applegate: Serial novels (54 in the main series, plus a number of related works) about a group of young teens who are given the power to change into any animal they can touch by a dying alien prince, and told to save the world from a parasitic alien threat. Originally released in the 1990s, this series is enjoying a renaissance due to its recent republication. Not to be missed!
Robert Langdon Series by Dan Brown: Gripping crime/thriller/mystery series about a symbologist who uncovers ancient and deadly plots linked to well-known places, objects, and works of art. The illustrated versions of these books are best - that way you can see for yourself exactly what the characters are looking at.
Eon Series by Alison Goodman: A girl disguised as a boy strives to become a Dragoneye - an honourable position in which one forms a bond with one of 12 dragons and thereby gains magical powers that allows him to control the weather and protect the land. Great girl-power story with awesome Asian influence.
Uglies Series by Scott Westerfeld: A teenage girl defies convention in a society where everyone is made "pretty" via a dubious operation when they come of age. Thought-provoking sci-fi YA quartet - my favourite was the last book, Extras.
Heir Apparent (Rasmussem Corporation, #2) by Vivian Vande Velde: Second and best in a series of books about full-immersion video games. A teenage girl becomes trapped in the video game she is playing after the immersion equipment is damaged. She must win the game (no matter how many tries it takes) or face a grisly fate. A great Groundhog Day-like YA novel for fans of video games.