Risk

Here is an old classic that don't need a lot of explanation since a lot of people may have already played the board game version.

Its a world domination game that take place on the world map with only 1 type of unit. I personally think that its the Risk ultimate game mode that deserve the attention since it has far more depth. Heres some of the added depth explanation: general unit that let you have bigger army and better tactical choice, fort, capital, port, weather affecting move and combat, terrain (like woods and mountain) giving an edge or a disadvantage, prisoner exchange, other map like America or Europe etc... formation choice in battle. disease, the rebels, add power up and you got a far more complex and challenging game.