I'd say Ironclads, by far.
Sea power isn't as important in this game as it could be, compounded by the fact that the AI has almost no sea warfare strategy. Hence, you simply can pass on ironclads and wait for the real warships to arrive on the scene.
Advanced Flight isn't that bad. Helicopters and (to a lesser extent) paratroopers can provide some real flexibility. And at this point in the game, cash shouldn't be a problem so spending a few gpt on unit maintenance for several helicopter and paratrooper units shouldn't hurt my finances. They can provide me some situational strategic options behind enemy lines.
Similar to my liberal use of bombard units, I simply don't like to fight as the AI do in wars. Going beyond enemy tactics (admittedly, in the case of the AI, it's not saying much), flanking them, and crippling their production are the human advantage against the the AI production and other advantages at higher levels. I'd only be handicaping myself if I just dismiss these units.
Having said that, I typically go for a few modern age techs first before going back to get or trade for advanced flight. I said the tech's not bad, just not among the more important ones.
