In your opinion, what would the archetypal spellsword use, heavy armor, light armor, or mage armor (alteration)? I know anyone can use anything, but which do you think best defines a spellsword?

I say Light Armor.

I'd probably say light armour, perhaps with flesh spells as well. I don't think there's any set definition - it's up to the individual character. As a rule of thumb, however, I think of heavily-armoured mages as Battlemages, and lightly-armoured mages as Spellswords.

Mage Armor (Alteration Spell) with either Light or Heavy Armor, I would more go with Light Armor since to me a Spell Sword is more on mobility. I might even use some unarmoured clothing as well.

From my perspective, I view Spellswords being donned in Light Armor. The Alteration spells can be a good boost prior to engaging but Light Armor is the primary goto.

Light armor, with the Armorflesh spells to bolster.