To offer an unorthodox choice, I'd say Echelon, if you're into futuristic aircraft. They all have cool names like the Wraith, or the Werewolf. The missions are challenging, but the dogfights are spectacular. You can enable ammo limit and even have infinite health if you just want to kick some ass, too. The requirements are minimal-- if it can run on my cranky old tablet PC, it can run anywhere.

Though, I haven't played any other flight sim, so I don't have any standards of comparison.