9600k or 9700k overclocked to 5Ghz+.. single thread clock speed > core count. radeon 7 or 1080ti or 2080ti vram is more important then outright gpu performance especially if you're running the high res/realistic texture packs. if you're not running 4k+ any 8GB card from the 1070 to 2080, vega 56/64 will work as well.. but spend your money on the peripherals more than the hardware.. a decent setup will easily cost you 3 grand+ just in that not counting the actual computer hardware.
also start looking at moving to x-plane 11 and/or prepar3d or DCS if you're into more military type planes.. MFS is on it's last leg. x-plane is working on moving over to vulkan api but no hint on when that might actually release.
As others said, Flight Simulator is all about single core clock speed. DCS is, as well, although recently they moved audio processing to a separate thread. Most simulators are going to be about single core performance because it is difficult to synchronize threads when you're going for accurate physics calculations. Right now the 9600K and 9700K will give you the best single core performance. Spend your money smartly on your PC so you have more to spend on peripherals.