Honestly, I don't think I can tell you what your favorite part of a film should be. This is a question wherein the answer will differ drastically from person to person. When someone asks what your favorite part is, they're asking your opinion. There's no hard and fast answer to an opinion question. There's also really no right and wrong answers to opinion questions.
That being said, are you talking about the 1976 film or the 2003 film? My favorite part in the 2003 film is when the mom, played by Jamie Lee Curtis, plays with the daughters garage band. One feature I think that makes the film enjoyable is that it playfully deals with a sensitive subject: strained parent/child relationships as they arise during adolescence.
An Amazon.com customer, T. Fisher has this to say about the movies, "This is a great movie, and my interest in it was rekindled after seeing the remake with Lindsay Lohan and Jamie Lee Curtis. While this has a quaint 70s charm to it, the remake is, in my opinion, a funnier and better-made movie. Production value is higher in the remake, certainly, and the jokes are more over the top. I think the original was the best movie they could make on this subject some 25-30 years ago, but they were able to bring more to bear later. I would give this 4.5 stars if the system allowed it -- just a hair below the remake. If you are only going to own one of them, go with the other version."