I don't think I need to provide an explanation. It's Harry Potter. Enough said.
The Ruby Oliver series by E. Lockhart is my favorite YA series of all time (so far). I love to laugh and all four books in this series had me laughing out loud. Although these books are incredibly funny they also deal with some very serious topics. I also loved the characters, especially Ruby who is both likable and relatable, and I thought the romantic relationships were done well. If you like contemporary YA novels then you have to read this series.
A Song of Ice and Fire:
While the fourth book was lackluster (with the exception of the last 20%), the first three books in the A Song of Ice and Fire series were fantastic, especially book three. These are long books and I'm a slow reader, but I finished each of the first three books within a week of starting them. The TV show is also awesome but no substitute for the books.