While i agree with @tofro s answer generally i take objection to "denn" being mere decoration.
The difference between:
Welche Art von Filmen schaust Du Dir am liebsten an?
and
Welche Art von Filmen schaust Du Dir denn am liebsten an?
is that the latter implies a similar question has had been answered by the one asking before, like i.e.:
Q: "Welche Art Filme magst Du?"
A: "Ich mag Filme über deutsche Grammatik. Welche Art von Filmen schaust Du Dir denn am liebsten an?"
In general, "denn" changes the mood of a question:
Was ist das?
Was ist das denn? (alt: Was ist denn das?)
The first is a neutral question, the second is a rhetorical question expressing astonishment, disbelief or even disgust.