After the first few missions a new threat emerges, the Shivan. Freespace perfectly captured the feeling of being outnumbered and outgunned. When the Shivan first appear, you're lucky if you can keep your ship intact and attempts to fight back result in scratching their paint if you're very lucky. This fear was replaced with joy as missions lead to captured technology, eventually enabling your fighter to pick up Shivan ships on sensors and develop shields so you can take more than one hit. Even when the tech starts to balance out, the game remains challenging, as better Shivan fighters and pilots are rolled out. Freespace is an incredible underdog story, which wasn't afraid to introduce unwinnable scenarios or change the rules on the player mid-mission.
Freespace also gets a lot of credit in my book for the sheer variety of missions they assigned you to and how something as simple as a routine escort mission could become more complicated when an alien armada shows up and you're expected to do recon on it. And the sheer variety of spacecraft gave options for every play style, whether you preferred the nimble Ulysses and maneuvering around your enemies or the superior Hercules fighter and a ton of firepower at your disposal, you had the option for it. Furthermore, I really felt like I earned my medals in Freespace and that I was an integral part in the war effort, even if sometimes it felt like I was the only one doing anything. While the sequel out performs the original in many ways and has incredible moments of its own, it's the original Freespace that most made me feel like I had to work hard for every victory and truly captured that underdog feeling.