This is my favorite show! Joss Whedon is one of the best writers on TV and my personal favorite. He's made other great shows as well - Angel, Firefly, Dollhouse - but Buffy is still my favorite. There are just so many layers to the characters and themes. Also, the musical episode is my favorite episode as well.
EDIT: Some interesting links for anyone interested in digging deeper into the show. The first is a group of reviews by a guy who knows the show well and looks into how each episode integrates and enhances the whole. There are spoilers in every review -
http://www.criticallytouched.com/buffy/. The second is spoiler free if you start from the beginning, with the exception of some of the comments, and also digs deeply into the characters and philosophies -
http://unpaidsophistry.blogspot.com/.