Hearts of Iron is simply the best and most definitive of the epic-scoped WWII strategy games available on the market today. It allows you to control any country of the era (including some VERY obscure ones) and guide it through perhaps the most tulmultuous period in history.
You control production, research, trade, who you wish to fight and when. You have some influence over the political direction of your country and also control your efforts in diplomacy and espionage. Research is somewhat changed from HoI 1 to make it less exploitable providing a slightly more historical experience. The addition of the Doomsday add-on pack provides for a lot more what-if options as well as providing an opportunity to press for world domination in the nuclear age which adds a whole swathe of new issues.
The learning curve for this game is fairly steep but the manual is fairly comprehensive if you're willing to dig a bit. Additionally there's a strong online community who are more than happy to help with any questions you may have.
In summary, if you're a fan of WWII grand strategy and don't mind putting a little bit of work in this game is simply a must have and will reward you in spades for your efforts.