Is there any objective, factual/rational basis for regarding, say, belief in Zeus, Ra, or Huitzilopochtli as more (or less) credible than belief in Jesus, Allah or The Flying Spaghetti Monster?

I don't see any.

All of them rest on blanket assertions that must be accepted "on faith" (that is, without any real proof beyond the say-so of human-authored "holy" books and human "holy" men) yet which don't hold up well to logical scrutiny. Most, if not all, claim to be universal and true from the dawn of time to now - yet none actually existed from the beginning, nor is universal. Each arose in its own historic time period, lasted for awhile and then eventually died out (as well happen in time to modern religions like Christianity). Each was a product of its regional culture and people - and frequently isolated to a geographic area as well as a specific historic period. Apparently, all the people who did not have the good luck to be born into that geographic area, or who lived hundreds (even thousands) of years before the "good news" appeared - are outta luck, as far as their salvation is concerned.

Does it make sense to you?

It doesn't to me... .