If you're a big fan of Civilization V, then you'll love Civilization V: Gods & Kings which adds more techs, units, buildings, Wonders, and playable Civilizations to Civ V.
Civilization V: Gods & Kings features 27 new units, 13 new buildings, 9 new wonders, and 9 new playable civilizations. Included among the new civilizations are the Netherlands (William I, Prince of Orange), the Celts (Boudicca), and the Mayans (Pacal the Great).
Perhaps most importantly though, Religion features in the game for the first time. This works using a new new "Faith" resource in which you're able to found your own religion and try to spread it worldwide. There's also a reworked Combat System that puts more emphasis on tactics and having a balanced army composition as well as Enhanced Diplomacy, Espionage and Foreign Affairs.
Finally, there are three new scenarios in Civilization V: Gods & Kings: the fall of Rome, the medieval era and a unique Victorian science-fiction setting.
If you're a big fan of Civilization V, then Civilization V: Gods & Kings is an essential expansion pack.