There are a million flight sims out there.I'm currently playing IL-2 Sturmovik 1946.It features planes from WW2, primarily German and Russian.
There's a campaign, quick mission builder and tons of single player missions.The real cool part about this game though, is the full mission builder.
With perhaps the most confusing and quirky interface I've ever seen
![LOL](images/smilies/icon_lol.gif)
The full mission builder is the shizzle.
You can put everything from stationary AA to moving trains and each AI component can be tweaked, right down to the time it does particular actions.
Building a full mission is a labor of love, but oh boy...the results are fantastic.