Victoria II is a real-time grand strategy game in which you take control of a nation in the "Long Nineteenth Century", guiding it to glory from the ascension of Queen Victoria through to the beginning of the Great Depression. You can control of almost any nation that existed during that time period, including established powers like Britain, "young guns" like the United States, and tottering has-beens like the Ottoman Empire, and guide them through industrialization, trade, diplomacy, war, colonialism, and political strife.
The game has much to reccomend it. Gameplay is subtly addictive, a lot like what happens with the Civilization series. You keep playing way longer than you should because you're always thinking "just let me finish building this cloth factory/colonizing this state/negotiating this alliance etc." The extensive historical research that went in to developing the game is another plus; history buffs (like me) can geek-out on playing through events like the Scramble for Africa and the American Civil War, and novices can learn quite a bit without even realizing it. Art and sound are also great, with loading screens that look like oil paintings and the soundtrack a combination of gentle waltzes and rousing military marches.
Unfortunately, this game does have a few downsides that keep me from being able to give it a perfect score. The learning curve is steep and the in-game tutorial is not that helpful, so you'll probably need to consult online wikis and YouTube guides to really get to grips with the game. The AI, while competent, is not always great, and will sometimes make strange or stupid decisions, like suicidally charging its entire army into a forified mountain province or building factories that instantly go bankrupt.
Overall, Victoria II stands out as a cult classic in the strategy genre, a detailed and fun portrayl or a historical epoch that tends to get passed over by gaming as a whole.