According to Guns, Germs, and Steel, what was the most important advantage that the Eurasians enjoyed at the time of Columbus?Chapter 18:...

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.