Favorite archer: Thanvare Payadag. Available everywhere in the east, has very good attack and range, level 1 MIC, very cheap. If you want to send your archers into melee, pick someone else for the job, though.

Subeshis, Mardians, and Caucasians are all quite good. Can't comment on the Scythians or Bosphorans - I have some, but the Crimea has been completely uncontested in my Hayasdan game.

||Lz3|| - Scythian units are found basically all around the northern half of the Black Sea, plus the Caucasus Mountains. For factions not named Hayasdan, I believe the standard method to acquire them is to take the Crimea.

On the "more fun troop combo" question - horse archers! Especially cataphract HA, though you're out of luck there with AS. You can get Parthians and I think also Dahae HA in the northeast.