Seleucids. I never really figured out how the world passed from Rome to Alexander, and who exactly was between Alexander and the "Persians" (i.e. Parthians) when I was ignorant but fascinated by history. The other factions are much easier to learn about (lots more resources) and therefore my fascination is now much less. Even now I couldn't really figure out how exactly did those Seleucid monarches rule their lands; and what kind of "land" were they ruling.