I'm going to take a crack at answering my own question. My answer has certain limitations, insofar as it is a purely mathematical, mechanistic construct. It also applies to starting "endowments" of military and territory, and doesn't allow for the fact that new techs may affect strategies (e.g. jet fighters make you build more fighters than otherwise). Nevertheless, something is better than nothing.
The least useful tech is super subs. It improves subs' combined combat factors (attack plus defense) by 25%, (five versus four). Applied to an IPC value of 8, that's a gain of 2 IPCs. And most countries have only on sub. I can't see spending 30 IPCs to research this. Others pay for themselves in about four turns (or less).
Likewise, the combined combat factors of jet fighters is improved from seven to eight. Applied to their 12 IPCs, the gain is about 2 IPCs. Most. countries have three or four fighters so the gain is 6-8 IPCs.
Economic relief reduces IPC costs of units by 1. The gain is 50% for infantry and 4% for bombers. Arbitrarily, I said that the "average" unit is a tank, for a 25% gain. That's a pick up of 6 IPCs (out of 24) for Russia, and 9 IPCs for the US.
Rockets give (one) anti-aircraft gun the value of a bomber. Essentially, your 5 IPC gun has been upgraded to a 15 IPC bomber for a gain of 10 IPCs.
Long range aircraft? Bombers and fighters are about equal in combat capacity (the first on offense, the second on defense, seldom the other way around). But bombers cost 3 more IPCs for two more zones of range. Using this value for the upgrade, and an average of four "planes" per country, the gain is 3 x 4 or 12 IPCs.
And heavy bombers themselves? In the original version, a heavy bomber is worth THREE normal bombers. Since the latter are worth 15 IPCs, the gain is 30 IPCs, way out of proportion to the others. Using the "two roll" rule, the pickup is more like 15 IPCs, at the high end of the "pack," but no longer over the top.
My preference is to use modifications of long range aircraft (plus 1 square instead of 2) for a gain of about 6 IPCs and the 50% better heavy bomber (THREE good rolls on two dice instead of four; strategic bombing averages 5.5 hits instead of 3.5), for an IPC gain of about 8. These changes would make "rockets" (ten IPCs) the best tech to research.