Best US 1 purchase in my mind is Two Carriers, 2 Fighters. Put both Carriers and a Fighter in W US, and one Fighter in Central US. On Round 2, you have a fleet (when evrything is moved toward Hawaii) that can begin to seriously contend with Japan.
Personally I like to purchase Carriers until I outnumber Japan - and if I’m Japan I like to outnumber the US. As a general rule of thumb, in the Pacific, if you outnumber your opponent in Carriers, and are in a SZ with an island airbase, you can hold that space. I said general rule of thumb, because the closer the US gets to the mainland, the more danger because of the dominance of Japanese airpower.
Yet another reason to buy two Fighters on US 1 as well… to begin inching closer to rivalling Japan in the air.