Darkest Hour is definitely a better HoI game than HoI3, despite its age. There's no unnecessarily complex OoB system that you'd need to graduate West Point to master, there aren't a billion shapeless provinces in sparsely populated areas, and although the map and UI are pretty dated they're comfy to use. In contrast to the dryness of HoI3 which has maybe half a dozen decisions solely for the purpose of railroading the AI into doing what it's supposed to do, DH has plenty of dynamic event chains, and it feels like you're really impacting the world. DH also has more of a sandboxy feel to it, with different outcomes every game, but in HoI3 once you've played each of the 3 factions you've more or less experienced it all. Then there's the timespan, with a grand campaign in DH potentially lasting from 1914-1964, 50 years, compared to 1936-1948, or 12 years, in HoI3. And last, but definitely not least, DH is tremendously easier to mod than HoI3, and so has a very active and skilled modding community. Which brings me to my last point:
Kaiserreich: Legacy of the Weltkrieg. The amount of detail put into this alternate-history mod for Darkest Hour is pretty stunning, and is definitely a justification all on its own for buying the game. If you have even a slight interest in alt-history, you'd be doing yourself a huge favor by picking this up and installing KR.
The base game is easily a 9/10, but Kaiserreich bumps that up to a 10/10.