1. IL-2 Sturmovik (2001)
This realistic WWII flight sim has slipped under a lot of gamers' radar, but it's well worth it for its combination of hard but satisfying gameplay. Tricky takeoffs are followed by lenthy flights to the battle, then lethal bombing runs and dogfights – and if you survive, you have to fly home and land properly. The online multiplayer of the early games was well ahead of its time, influencing modern sims such as War Thunder, and the series continues to innovate.