Falcon 4.0 is probably the most realistic combat flight simulator. Lock on has features that Falcon doesn't have and vice versa, I would recommened both of them but personally I like Falcon 4 more.
If you have plenty of time on your hands and want to learn to fly properly you should check out the FreeBirds website, it provides multiplayer environment for training & combat. There are some really good pilots there and you can learn alot from them.
http://www.freebirdswing.org/
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woops, this was supposed to be in reply to the 'Good flight sim' thread.