X-COM: Terror from the Deep

This is the game I played the most growing up and it still holds a special place in my heart. It's mostly a reskin of the original X-COM: Enemy Unknown, which is why most people don't rate this one as highly. I wouldn't recommend anyone playing it now over the modern X-COM games, but at the time it was stellar. Like most X-COM games, it has two game modes: managing the X-COM global strategy, and running a team as they battle with the aliens. Really fun and exciting for me at the time.