The best flight sims out there are actually a fair age now.
If World War 2 is your bag, try the excellent IL2: 1946. It's essentially the entire IL2-Sturmovik series plus all the expansions in one handy pack. The difficulty is massively customisable, especially with the latest patches installed. At its hardest settings, it's positively sublime. Each aircraft (and there are literally 100's) has its own unique traits and characteristics. Learning the strengths and weaknesses of your mount is important to victory, and the game can be punishingly hard at times. It's over a decade since the original appeared, but 1946 is supported to this day by the community and some fantastic mods are available to make everything look all shiny and nice on newer machines. There are also endless amounts of fan-created content and campaigns. It's possible to fight in every major conflict from the Russo-Finnish wars of the 30's through the invasion of Poland, the fall of France, the struggle for North Africa, the Eastern Front, the Pacific Theatre and literally any other major (and many minor) actions you could ever think of. There are also lots of fictional and semifictional campaigns out there.
For Cold War era jets, try Strike Fighters 2. It's the least hard core title in this little list, but it does what it does very well. The handling model is pretty forgiving, although you can still get into trouble on the hardest settings, but it doesn't feel too simplified. With the advent of homing missiles, radar interceptions, SAM's and supersonic jets the workload during a fight goes up, and the game takes just enough of the load off your shoulders to allow you to focus on the combat. The combat can be frustrating at first until you realise that the US jets you fly are generally heavier, but faster, than the Soviet-built jets you run up against. With the addon packs installed, you can also fight a number of real life and fictional conflicts set in Europe, the Middle East and Vietnam from the advent of all-jet combat in the '50s through to the advent of the modern AWACS controlled battlespace and the dawn of smart weaponry in the 1980's.
For more modern jets, and a much, MUCH more hardcore experience there's the magnificent Falcon 4.0. Over 15 years old, and another still supported by the community, it's never been bettered. It's a very, very accurate recreation of the Block 40/50 F16-C Fighting Faclon. The F-16 is the only flyable aircraft in the game, but there are many varied types, both NATO and Russian, that' you'll see and interact with during the game. Take part in one of six fictional campaigns (3 set on the Korean Peninsula, 3 set in the Balkans around the former Yugoslavia), and choose from one of three time frames to fight them in. The default era is 1998 (when the game released). You'll get less sophisticated weaponry, and earlier model F-16's might not have one or two minor features installed. On the other hand the standard of enemy will also be lower, with the enemy flying older types and also deploying less sophisticated weapons. Moving time forward to 2005, or 2010 will put more advanced aircraft, weaponry and anti-air defences into the battle. It also comes with a 700+ page manual and can't be learned in a day or two.
Elite looks pretty promising as a space combat effort although I haven't played it. As you can tell from the above, historical combat is kinda my thing...
