I love Meg Cabot, its always a fun read, quick, fluffy and entertaining. I read The PD, The Mediator, All American Girl, Teen Idol. Her Adult books are good too. I really liked The Boy Next Door. She's really mastered the diary, IM and e-mail story telling, even if her writing has a pattern.
The Sisterhood of The Traveling Pants Series is good too.
Just thought I'd jump in and add my thoughts. Most of these I've already read, and have them on my shelves. But I think Diane Duane's Young Wizards series is pretty good for young adult readers, especially boys since it has both a male and female main character. Although compared to the Harry Potter books, they were out in the eighties.