In Chapter 18, Diamond does not pick out any one advantage as the most important. He simply lists advantages. He splits these advantages into ultimate causes and proximate causes.
The ultimate advantage that the Eurasians had was in agriculture. Here, Diamond says the "most glaring" difference was in the number of large domesticated animal species that the Eurasians had. They also had advantages in crops, but the animal advantage was more clear cut.
The proximate advantages the Eurasians had were germs, technology, political organization, and writing. Diamond lists all of these as "most important" among the proximate causes. He does not say that any one of them was more important than any other.
If you are required to say which of these was the most important, I would argue for the differences in agriculture. These are ultimate causes, meaning that they helped to give rise to all the proximate causes.