The position immediately following the first operator is typical for an adverb.
Forms of be, do, and have are considered operators. Modals are, too.
It is simply the best computer we have.
But adverb position is not usually strictly fixed, and you will also hear variants such as yours:
It simply is the best computer we have.
I would not say that it is wrong -- just that it is a little different from the norm. 
CJ