“Filler” is very subjective. A very long chapter going on a tangential introspective mourning thoughts may be viewed by some, as it “doesn’t move the plot forward”, but it can be the most pungent point of emotional connection in the entire novel for others.

Sometimes going on talking about minor characters routine is kind of the main point of an author’s intention, but it may read as “filler” by some.

Or something like Brandon Sanderson books, where his fans enjoy spending more time in that universe, but make their novels very long.

"A Canticle For Leibowitz"; "The Stranger"; "The Fall"; "Childhood's End"; "Riddley Walker"; "The Martian Chronicles"; "The Sea of Tranquillity"; "Animal Farm"; "Catch 22"; "The Catcher in the Rye"; "A Clockwork Orange"; "Dr. Strangelove"; "To Kill A Mockingbird"; "The Little Prince"; "The Unbearable Lightness of Being"; "Lord of the Flies"; "Charlotte's Web"; "A Tree Grows in Brooklyn"; "All Quiet on the Western Front"; "The World According to Garp"; "Invisible Man"; "Flowers for Algernon"; "Sophie's Choice"; "The Road"; "All the Light We Cannot See"; "The Goldfinch"; "Atonement"; "The Curious Incident of the Dog in the Night-Time"

"a wizard of earthsea" is one of my gold standard novels in that respect

What is "filler content"?

Because from how I understand it I'd argue there is no such thing as "filler content" in novels.

To the other great nominations, I would add:

"The Hitchhiker's Guide to the Galaxy"