That's a pretty broad question. As a general rule, it would be hard to say what any country would be like if we go with an ancient POD. One might imagine a Persian Empire that continues to exist to the current day, and which still (or again, after several periods of decline and resurgence) extends to roughly the domain once held by the Akhaimenids. However, we can't tell what such an empire would be like. From an OTL point of view, its culture and society might not be what we consider desirable.
If we assume that "best" just means "most succesful on its own terms, regardless of the opinion of any others", then I would say such a Persian Empire prevailing would indeed be best. I'd consider it better than any Muslim Persia, even if that were to be of similar size and wealth, simply because a non-Islamic version would retain more of Persia's own ancestral culture. I think that would be definition by "better" from the perspective of its "Persian-ness". I also think that preventing the Sassanids and their particular religious preferences would be a good place to start looking for PODs. A less divided Zoroastrianism, with less forced imposition of the ruling elite's ideas (and correspondingly less public opposition to those ideas) would be more stable and viable.
Considering the size and resources of such an Empire, I can easily see it as a power similar to OTL's China.