Headquarters (XCOM: Enemy Unknown)
Your secret underground headquarters are a vital part of your overall experience and success with XCOM. Choosing whether to build a research lab or engineering bay in your limited spaces can ultimately be just as crucial as deciding whether to take a risky shot on the battlefield, and higher difficulties require you to plan your base construction out perfectly.
The expansion, Enemy Within, also brings back the base defense missions of the original games, giving you an even greater connection to the home you've built for your soldiers as they defend waves of alien attacks.