Civilization Revolution will set you back just 69p / 99c now. And while we weren't impressed with the iPhone version [buy], the iPad version [buy] is an improvement over its little brother thanks to its less fiddly controls.
The Gold Award-winning XCOM: Enemy Unknown, on the other hand, is one of our favourite games of the year. At £6.99 / $9.99 [buy], it's an absolute steal. If you haven't picked it up yet, it comes highly recommended.