If you are prepared to wait about ten days, the best one is likely to be the new Battle of Britain-themed one 'Storm of War' made by the people who did IL-2.
Other than that, you've got:
IL-2 (in any variant, but probably 1946 version), best all rounder and probably best value, some questionable single player AI, but brilliant against human opponents. Models virtually every plane ever built including some that in fact were never built. The '1946' element is more like a fantasy than an extrapolation of real prototypes that would have made it into action by that year, with things like a V-Tailed Me262 instead of real, planned - and existing - variants of the Me262, such as the version with a bomb aimer in a clear nosecone, and a hybrid BMW-powered rocket/jet interceptor version which did actually fly. It would have been better with things like that in it. Nevertheless, every flight sim fan should own this.
CFS3, much maligned, but very customisable and fun, pushes the envelope in terms of interesting gameplay, will also allow you to use the Over Flanders Fields freeware WW1 add-on, which is superb.
BoB2, a bit quirky, and not easy to play a pilot campaign, but has a lot of depth and allows you to play either side as a controller as well as a flyer, missing some very important aircraft types from the Battle though, and not easy to fly on realistic settings.
B-17 2 The Mighty Eighth, Bloody great game, very immersive, you can play as a German fighter pilot if you like, or an escorting Allied fighter pilot, or all three if you like. Best implementation of missions for any WW2 flight simulator ever, requires genuine navigation and piloting skills that other flight sims do not model, such as wind-drift on bomb runs etc. also models the human side of things, like CFS3 does, and will allow you to manage an entire bomber wing, as well as just a single plane and crew. A much-underrated classic that is still very playable.
Chock