Red Baron is one of the greatest World War I flight simulators. It features a total 17 different planes. You can play this game in Story Mode or in Head-to-head flight mode. In the latter you will fly against some of the biggest names in the First World War, including the Red Baron himself. If you want to, you can also change the planes for each pilot, so that you can see how well he handles a British plane. Each plane has a different layout, abilities and weaponry.
In story mode, you get to choose which side you will fly for. Obviously this will limit the types of planes you can fly, but it is in my opinion better than flying head-to-head. You start the story as a flight leader, where you command a tiny force of planes. This group will increase or decrease in size depending on your piloting skills. The AI for your friendly aeroplanes is very well made, and they will manage without your help. All you have to worry about is staying alive.
The engine used for the game is also a good piece of software. The scenery, flying and maneuvering are all nice to watch.
There isn't much more to say about the game. You receive missions. You fly. You kill, and hopefully you'll stay alive. Try it.