personally i went for 4 (armory), 4(oil well) so i can build airport and 4(library)
i guess u have no choice in starting by those, if u dont upgrade armory and library, ur resources will accummulate while u have no free workers, so u should upgrade them to spend on researches and upgrades while ur buildings are upgrading
oil well is good to prevent max oil accummulation, and for airport of course, none of IA oil requirements can be met by lvl 1 oil well, so u will eventually be forced to upgrade it
then upgrading mills (food stores) for armory upgrades, if u don't have notre dam u will have to upgrade 2 of them to get 7 mil store capacity(highest cost 7mil food), if u have it u will have to upgrade 1 only to get 6,5mil capacity (highest cost will be 6,3mil food)
upgrade gold stores (2 is mandatory)
my plan in general is going for airport then upgrading it to lvl 2, alliance gate, bringing 4th catapult to EA level so i can start 2 of them upgrade to mortars and spare 2 for defense, same will be done for anti tank guns (ballista), then barracks 1 by 1 (takes 8 days -_-)
least priority for me is resources buildings, factory (not planning to use MK tanks now + i dont use factory units so much), garrisons and stables (probably 1 of each is enough after upgrading mortars and anti tank guns)