My favorite is MS Combat Flight Simulator! When you get bored with the 'stock' planes that come with this game, you can download tons of different planes from the internet, you can even design your own! You can even fly planes from other periods against the WWII planes. I can't wait until the newest version comes out. It will be based in the Pacific instead of Europe. It is due out this fall, but I can't remember the name?
I have Combat Flight Simulator and European Air War. Both are good in their own right. Both handle the flight models well and both allow selecivity as to the "realism" you want to use - EAW even has a setting for vibrations from the engine and guns as well as hits on your plane. I like the plane selection in EAW better as it includes the P38 Lightning (my favorite). Nothing like bearing down on your enemy with 6 machine guns and two cannons. Can you say SHRED!!!Combat FS has well rendered planes (the skins anyway as the object itself is a little blocky for my taste) and mountains in the landscape, but EAW has beautiful planes and tons of ground objects to blast away at.
All in all, I'd have to say EAW is my favorite because it just looks better, and both the instant action and campaigns are more realistic. Thats the one I keep going back to.
the bset i cna think of is Janes World War II sim....janes combat sims makes the best sims out there...i have almost all of them (although i'd rather be firing sidewinders and pheonixes than 20mm..cannon shells anyway!)