TL;DR; Buy a carrier, or three, then buy a heap destroyers to flesh the thing out, and as time goes on, focus on bombers.
You will need about 1 hit point per hit point that the enemy can bring, to deter an attack. The problem is, that in a 50/50 (or worse) battle, the enemy may still attack you because he knows that losing all your TTs is devastating and can cripple the USA/UK/Japan especially. Even if the enemy suffers massive losses, he killed your only fleet that it took multiple turns to develop, so he only needs to kill your last defender to “win”, no matter the cost to his forces.
Carriers: Best Choice. Flexible on fleet defense and on the attack. Downside is that these take the highest investment of IPCs for the lowest attack/defense power, because you get both. A fleet of primarily high cost units (carriers+planes) is surprisingly weak (since carriers add nothing to attack power) You also get a bunch of free air to start that can just land on new carriers, so that makes the expense a bit lower in the Global game especially.
DDs: Too costly, but indispensable. Once you have a BB and a CV in a certain fleet, adding 3-6 destroyers (and then more, ad nausea) gives you the most extra hit points for the lowest cost, which keeps your high attack power units (bombers and fighters) alive longer, which is the key to winning a stack battle.
Cruisers; Not worth it. Don’t buy these.
BB; too expensive, but the soak hits make these the depth of your fleet. 1 is probably enough, per fleet, more is gravy.
Subs; Most efficient attack, but totally inadequate on defense (even as cannon fodder–all your opponent needs to is attack with air without a DD and these do nothing). Buying a few is a great power booster and deterrent, but these cannot be relied upon.
Transports; since these do nothing in battle, overbuying these is your most common undoing. If you cannot maintain the 1:1 hit point parity, favor warships over TTs.
Tactical; Because they are dependent on another unit, and you have a bunch of these to start, fighters and strat bombers remain more focused choices. Don’t buy these so much as preserve the ratio 1:1 of fighters to bombers that you start with.
Strat bombers; fastest way to get into the action, most flexible overall unit in the game. Downside is that these add nothing to the defense of your fleets, and create new liabilities if they all land together.
buy surface units in earlier turns, and bombers in later ones, they can all attack together.
If you are uncertain you will win on the defense, don’t bring all your transports to 1 place (don’t put all eggs in 1 basket)
First; get a grand fleet together (DD CV BB TT). Cruisers are fine, just extra.
Then, buy more carriers up to how many planes you have at your disposal (with the US, its 3, Japan 2-4, Germany, 1)
Then, buy more DDs as cannon fodder (4-8 is plenty)
Then, buy Stratbombers as a follow-on force (USA) (4-8 is your lightsaber)
Buy the other stuff sparingly, and in deference to the above priority
then, cross and blow up your opponent
Strat bombers with airbases can fly vast distances, and the enemy can build bases as a surprise. If you sense that you are too weak to sally/cross, it is often because you are building too many nonwarships too early and units that crowd up at your capital and wait aren’t accomplishing anything.
EDIT; If your team can buy Borg Cubes, do that instead, they’re virtually indestructible.