Xplane and msfs are the canonical ones. The current msfs is actually very newbie friendly. Great guided tutorials and the complexity and difficulty is hugely scalable.
I enjoyed aeroflyFS also however as a simple pickup and fly.
On the combat side the top games are War Thunder (which I don't recommend due to it's microtransaction model), DCS World (flawed and expensive but still fun), IL-2 Battle of Stalingrad (good WW2 game) and Falcon BMS (excellent, but focused on the F-16 and has a small community)
Since FlightGear only comes with one aircraft by default, here are some suggestions for very high-quality aircraft to download:
- TL96. Great little ultralight which will get blown around with the slightest gust (be careful on approach!)
- Piper J-3 Cub. Lots of things to customize on this one, with 'no doors' being an option for those who like the view!
- Cessna 182. Just as good as the default 172 but with some more horsepower.
- Piper PA28. Basically as good as the C172 and C182, but with a low wing to distinguish its performance (it has a tendency to roll into the turn).
- Cessna 337G. Really fun to listen to!
- Lockheed Super Constellation. Beautiful late-50s propeller airliner. The autopilot is touchy, the landing speed envelope is small and the engines are wont to burst into flames, but it's so rewarding to get it right!
- Cessna 208B. One of my favourites - turboprop, high-wing, lots of cargo space. You can fly FedEx up and down California in style with this one :)
- Boeing 707. Very fast early jetliner which is a pleasure to fly. The flight engineer's panel is well simulated.
- Tupolev TU-154b. Amazingly detailed, just like the 707 is. Hope you can read Russian ;)
- McDonnell Douglas MD-11. There are two variants of this available for download, so make sure to get the one with the higher rating. The simulation has some weird glitches but it's a good aircraft overall.
- Boeing 777. Strong contender if you like the 'heavy iron'.
- Airbus A320. Excellently simulated, probably the best there is for FlightGear. Almost every switch and button works exactly as described in the real-life manual. Trust me, I have the real-life manual and I've tried :)