Meg Cabot has a unique writing voice; it never gets old. I have read all of her YA books except the Mediator series, and I have enjoyed them all. Her protagonists range from average girls to princesses, yet each character is someone with whom the reader can relate.
Note for parents: some of Meg's books talk about sex (NOT graphic), so that is something parents may want to know. (The Princess Diaries series is the first series by Meg that I would suggest for cautious parents.)
2. Jen Calonita
Jen Calonia's character in the Secrets of my Hollywood Life series is down-to-earth yet fun. She's a nice girl who lives a glamorous life. I like that combination because many of today's girls believe you have to be mean to get ahead or to be popular. This series shows that niceness does win in the end.
3. Melody Carlson: Christian fiction author
Melody Carlson's one of the few Christian fiction writers for teens. (Kristin Billerbeck is coming out with a YA book, and I can't wait!) The TrueColors series can seem a little preachy. In each book, a girl deals with a problem (alcohol, bad boyfriend, tough home life), and she eventually finds her way to Christ. The Carter House series is about several girls, each with her own set of unique problems, living in a boarding house. The Diary of a Teenage Girl series is written in journal form, recording the thoughts of a girl as she goes through high school.