I love Sarah Dessen. I have read all of them, I think, except for her most recent.You start reading her books, and you cannot put them down. They're really good, and everyone I know who has also read them thinks the same.
Most of them have the same sort of romance aspect to it, but each book deals with different sort of problems that teenagers always go through. If you are a teenager girl, you'll be able to really relate to them.
My personal favourite may be Just Listen, but all of them were really close. I don't think I can really say which one is the best though, because everyone has different opinions. I may think Just Listen is the best, but some people may hate that book and think The Truth About Forever is the best, you know?
And for the one you should read first? I say you choose. Go the library or bookstore, read the back of all her books there, and then choose which one interests you the most.