I've been googling this question today, and didn't find any answers, so I figured I'd consult the wisdom of the crowds that is the Bay12 Other Games subforum.
To my tastes, the best thing about HOI4 is the way production lines are implemented. In particular, I appreciate that it provides a sense of verisimilitude by givi fng your economy inertia... you can switch from producing one thing to another, but that loses you efficiency, so immediately switching all of your production from ye olde Tank 1 to whiz-bang Tank 2 often isn't the best choice if you're in the middle of a war. Similarly, if your strategy calls for overwhelming air superiority, you need to start building those planes well before war is declared.
I've been pondering whether any other game has implemented production lines in a similar way. The only game that comes to mind is War in the Pacific where, playing as the Japanese side (but not when playing as the US), you manage both plane and engine manufacturing allocation... this leads to interesting decisions around using inferior plane designs because they use an engine you have existing production for, because you can't always afford the efficiency hit (and/or resource cost) of retooling the engine factory.
WitP:AW was released in 2004... HoI4 in 2016. Are those the only two strategy games of the past 20 years that have nuanced production of this sort? (Even Aurora, which is often rated as the most complex space 4x, doesn't have production economies of scale like this, though it has some similar notions elsewhere.)