WW2 era
IL2 series is the best by a long shot for many reasons, anyone of them is better than the rest and they (Forgotten Battles, Pacific Fighters Aces Expansion pack) are played on a joint instal at SGS, accept no substitute, the rest are all rubbish. I say again rubbish.
Modern jets,
Nothing comes close to the complexity of Falcon 4 and the new F4Allied Force is good. Its an old sim that is continually revamped, the graphics are dated but dont let that fool you, the avionics and weapons systems are light years ahead of anything else. It will take you months to learn to fly this sim anywhere near properly, its great.
Also LockOn or Lomac (Lock On Modern Air Combat)
Has all the eye candy you could ever want, but you need a beast of a machine to run it at medium to high settings. This just gets worse in 'busy' missions.
Its got good flight models and is not nearly as complex as Falcon so if you are wanting to go jets, then this is probably your best bet coming from Janes. Its really good fun, but you will still need to put effort in and learn the onboard systems, its not BF2 k@k@.
Oh ja, dont even think about playing anyone of these sims without a descent joystick.
If you want to fly buses, go MSFS2000andwhatever, not a fan.
MS has a definate place in the market though for procedural based flying, the attention to detail of flightplans, atc, etc is really good, but the rest is just plain boring.