The best simulator, hands down, is Microsoft Flight SIm 200X (newest... 2004 PRO is the newest still, IIRC)
AFA war planes: there are a lot of good ones out there.. when you say "best", though, do you mean most realistic (though possibly less "instant fun"), or most "shoot 'em up" style, though less physics involved?
Red Baron is a nice shooter that takes little in to account as far as physics go, has a nice storyline (though linear) and keeps you rather entertained through the use of "stunt" tactics you can perform to gain experience point, while fighting. Though slightly dated, now, it still has good fun to be had from it, and can be found for $10-15, I would beleive.
REALISTIC one:I would, again, default to Microsoft Combat Flight Simulator (there are multiple "eras") such as European Skies, and World War II, etc. Plane handling, and mechanics are top notch. Combine this game with a QUALITY flight stick and you've got a REALLY interesting test of skill. (same goes for MSFS2K4) High-Res gameplay, and not too much of the "video" intros that keep you dragged down waiting to play. Amateur mode give the ability to remove the need for engine trim (oil/fuel) adjustment, prop speed/pitch manipulation, and other "little things" that would normally be required to ACTUALLY fly a plane of that type.
Ray