This is, of course, a matter of opinion. My own opinion is that all of the chapters have some strong arguments but that the chapter that is strongest overall is Chapter 17.
One strength of this chapter is the way in which Diamond uses both archaeological and linguistic evidence to show the direction and order in which Asians moved out into the Pacific islands. I also am persuaded by his argument (similar to that in Chapter 2), that the environments of the islands had a great deal to do with what sorts of societies arose on those islands. Finally, I agree with his argument about the reasons why the migrating Austronesians were able to displace most of the native peoples they encountered. This argument is particularly strong given that the case of highland New Guinea gives an example of how native cultures that already had agriculture were able to resist the Austronesians.
For these reasons, Chapter 17 seems to me to have the strongest arguments of Part 4.