it depends, what strategy you are following.
germany:
some people buy a carrier. because it is not sufficient to defend itself you need company, mostly a destroyer and a submarine. this put into the seazone 112 where a battleship, a cruiser and a transport are parked.
but, because sealion doesn´t win any game i prefer to buy three submarines and a bomber to establish a naval threat in the west.
russia:
most buys are 9 infantry, 1 artillery and a tank every round.
japan: a naval base and a transport, saving some ipcs or three transports and saving ipcs as well.
uk-europe:
mostly infantry. depends from the outcome of the first round of the german player.
uk-pacific: a carrier or men, depens from japan´s turn.
italy:
i lean towards buying nothing. just moving and attacking to secure good positions to invade africa via egypt.
us: here it depends from the decision where to go first: europe or asia. there you place your buys. i suggest for both theaters 4 submarines, a carrier and a bomber to place in one theater.
anzac:
same as italy. no buys but strategic movements to secure NOs in the 2nd round.
china: infantry as much as possible.