Hearts of Iron 3 is still the best WW2 grand strategy game ever made. It does have a number of issues though, especially relating to difficulty/complexity when it comes to new players, and the learning curve probably puts a ton of people off the game. Hearts of Iron 4 attempted to remedy that with an easier tech system, friendlier UI, simpler production etc. but it ended up falling short. The arcade-like nature of the game took a lot of the good parts of HOI3 out of the game, giving people less chance to micromanage on the frontlines, or create very detailed unit types.
This is where Darkest Hour steps in. Released 2 years after HOI3, it bridges the gap between 3 and 4, and shares most of the good qualities that both of the games have in on package. There are less provinces on the map, making tactical decisions easier to wrap your head around, but it isn't so small it removes challenge/opportunity. The political system is extremely stripped back, which I don't mind as it seemed kind of arbitrary and not fully functioning in 3. Technology/research is more in depth than in both of the other games and is essentially perfect.
The game also spans a much longer period, with the ability to play from the beginning of the first world war, or starting at the very end of the second, with a 28 year long campaign. Of course, you can also choose to start in between those dates.
It is currently £2 on Steam, which is completely absurd for how good this game is.
I'm not sure, could have been.
All I know is one moment I was invading Russia, the next I was signing the Treaty of Versailles.