H'okay.
Civilian:
FSX - Xplane is very good and up and coming, but doesn't quite have the legacy backing that FSX does, however I'd definitely keep an eye on it. FSX has the upper hand with the PMDG series and Accusim series which put the 'sim' in simulator as Skybird will attest to.
WWI:
Rise of Flight/Over Flanders Fields - I'd put both ROF and OFF at level pegging. I haven't played OFF since phase 2 (it's one phase 4 now IIRC) but it was quite compelling back then. What OFF lacks in graphics it makes up for in immersion. Rise of Flight is stunning, but the campaign is a bit stiff and there's just something not quite there. It's hard to explain, but it's a critique which has been levelled at quite a few simulators that have come out from the East of Europe over the past few years.
The primary advantage that ROF has over OFF aside from graphics is the price, OFF is quite expensive and (possibly) requires Combat Flight Simulator 3 (you'd have to check on that to be sure, they might have weaned it from it by now) whereas Rise of Flight can be downloaded for free with two aircraft and you just pay for whatever aircraft you fancy to fly. The AI aircraft are all there to battle against from the start.
WWII:
Battle of Britain II: Wings of Victory - Without question the best simulation of the Battle of Britain ever made. It's primary drawback is that it lacks multiplayer and the graphics are a little dated, but it makes up for it in leaps and bounds in every other area. The primary draw is the ability for the sim to model hundreds of aircraft in the sky at the same time, something that most simulators struggle with. When you come out of cloud at Angels 15 to see a whole box of He-111s in front of you with Spitfires and Bf-109s swirling around like mosquitos above it, then you get an idea of the scale of the battle in 1940. There's also a very in-depth campaign that you can fly in. Highly recommended and still being updated by a dedicated mod community that have done terrific things to it (including putting in every single squadron and staffels individual paint scheme, so if you recognise Adolf Gallands paint scheme on a 109, chances are that's the sim's Adolf Galland that you're running away from).
IL-2 1946 - Dated, the vanilla has some very questionable flight models and the AI is a cheating bastard, BUT, it's a very capable online engine and the scale of the theatres and aircraft in the 'sim' is unrivalled. If you want to fly in the Pacific for one mission and over Berlin the next, you can do that. If you want to do it with friends from all over the world, you can do that too. The TD patches are essential and you'll really want to get one of the modpacks, we currently use HSFX, and I recommend that.
Cold War/Modern:
Falcon 4.0 BMS - The original Falcon 4.0 updated for a modern era, it's graphically beautiful but has by no means been dumbed down. The campaign is very well made, one of the best in the sim world, and the AI is no slouch at all. The downside? Getting a copy of the original Falcon 4.0 became an expensive exercise as soon as BMS came out, on ebay copies can range from $40-100 which is rather steep for such an old game, but if you have a copy of the old Falcon, it's well worth getting BMS.
DCS Black Shark/A-10C - Beautiful and brutal. Like Falcon 4.0, this is something you'll spend a lot of time reading manuals for, likewise watching tutorials. The Black Shark is a little bit easier to work with than the A10C (In my opinion anyway) but be prepared to curse Kamovs twin rotor design a lot as they suddenly decide to interact with each other in a manner that you did not intend them to. DCS World is free and comes with a Frogfoot, so you can get an idea of the flight region, but the Frogfoot (like the Flaming Cliffs aircraft) is not up to the standard of the A10/Black Shark/Huey in terms of clickable equipment and detail. The downside is that like Rise of Flight, the single player is a bit...bland...compared to the likes of Falcon 4.0, but graphically it's beautiful and as a sim it's top notch. Avoid Flaming Cliffs though, the aircraft are beautiful and nice to fly online, but they have a nasty habit of rereleasing it as an updated version periodically and making the new DCS World completely incompatible with the older versions. For example, FC2 and Black Shark 1 are both outdated with the current DCS World...hopefully this is something that won't happen any more now that they've got DCS World sorted, but it's still something that caused a bit of upset in the Flightsim world (not that it takes much).