Since in AV is much different then 2 CH. If 2CH then #9 and #4 for sure.
In Movie, since we already spent most through eye, sound is only about 30% total for the movie. Movie reproduction is all about feel not detail in sound or how it really sound true.
Subwoofer can really add the feel of the whole picture. also all vocal come through center channel. this is also a must change.
These 2 items for sure have the most impact for 5.1 or 7.1 movie experience.
Hope this help.