In 2001, the #1 most-challenged "book" in the United States was the Harry Potter series by J.K. Rowling. At that point in time, it would have meant the first four books: Harry Potter and the Sorcerer's Stone (renamed from the original Harry Potter and the Philosopher's Stone), Harry Potter and the Chamber of Secrets, Harry Potter and the Prisoner of Azkaban, and Harry Potter and the Goblet of Fire.
Reasons for challenges in 2001: anti-family, occult/Satanism, religious viewpoint, and violence
In 2002, Harry Potter again led the list. Same four books.
Reasons for challenges in 2002: occult/Satanism and violence
In 2003, Harry Potter slipped to second place, despite the release of Harry Potter and the Order of the Phoenix.
Reasons for challenges in 2003: occult/Satanism
As I've mentioned before, I've read the first Harry Potter book at least a dozen times now, and Chamber of Secrets and Azkaban at least 10 times each. If forced to pick one of the seven books as a favorite, mine would probably be Azkaban, in fact, although it's a tough call. I've read each of the successive books a few times less - so, something like 8 times for book 4, about 6 times for book 5, 3 tims for book 6 and, I am somewhat chastened to report, a mere twice for book 7. I sense a mid-winter rereading binge might be in order.
I cannot think of a single instance involving Satan or any of his minions in the entire series. Nor can I find any evidence of Satanism. There are good witches, and bad witches, and people who, like Dorothy, are not a witch at all (although usually they are not the best sort of people, since most non-witches depicted in the books are not particularly good - think of the Dursleys, for instance, or Filch, who was a Squib; Hermione's parents, who are dentists, are spoken of, but appear only briefly in Chamber of Secrets.
Do the books include occultism? I'd say yes, in the best possible sense: they concern "the belief in or study of the action or influence of supernatural or supernormal powers". As do the Superman or Spiderman comics, the Narnia books, and more. Do they promote worship of dark forces? Not unless the reader is peculiarly warped.
These books are all about friendship and loyalty and trust, about light overcoming darkness, good triumphing over evil. They are about self-sacrifice and hard work and dedication, about setting one's priorities properly - Hermione loses her singular dedication toward getting great grades, Ron decides that friendship and loyalty and love are more important that individual glory, Harry learns that being an orphan doesn't mean that he has to be alone and without a family, Neville learns that hard work and dedication can raise one up from being a rather hopeless case to being a respected member of society . . . I could go on, really, with other characters, including some of the adult ones, but I won't. All of those lessons are the sorts of things that most parents want their children to learn, and setting them in a fantasy realm is a way of making those lessons accessible without them being completely overwhelming. Not that they are lessons that Rowling set out to teach in the first place - they (and more) are, rather, the take-aways that come from the stories she has set within the tremendous alternate universe that she has created.
As Dumbledore told Harry in The Chamber of Secrets: "It is our choices, Harry, that show what we truly are, far more than our abilities." This includes our choices as rational beings to stand up and say NO when book challenges or book bans come along. As Mad-Eye Moody is fond of exclaiming, CONSTANT VIGILANCE! is what is required to ensure that we remain an open society, and it includes just saying NO to book challenges and bans.
Incidentally, this does not mean that any parent has to say "yes" to any and every book their own child is reading. Parents ought to do what they think best when it comes to books their children are reading, just as they ought with respect to TV or movies that they watch, friends with whom they play, activities in which they participate, etc. Parents can and should monitor their own children; they ought not, however, try to prevent other people's children from access to reading materials, any more than they ought to slap someone else's child for carrying on in the grocery store.