X-Plane 12… Only because I seem to be having a bunch of problems with MSFS2024 today and have to reinstall.
Joking aside, have fun with the new computer.
That’s a pretty broad question, for computers there is hundreds of third party software addons. Figure out what you like to fly, commercial airliner, general aviation, helicopters, military aircraft, hot air balloons etc..Then search for a third party aircraft to meet your needs. Then maybe software to enhance your experience with that aircraft.
Maybe search for a local airport that you might like to fly out of and see if there is third party add on for it. Flightsim.to is a good website with lot’s of free addons. Scenery, aircraft enhancements liveries and more.
If your running MSFS2024 most devs are working on compatibility fixes right now. If your flying on MSFS2020 then there is a ton of stuff out there.
If your going to fly in vatsim you’ll probably want a Navigraph/simbrief subscription. For flight planning and charts, especially if your going to use commercial jets. the jeppesen charts are almost a must have. But there is alternatives to that as well.
I also like a little program called FS ATC Chatter. Which gives you background Pilot/ATC chatter for just general flying. you might want to look into some of the new Ai type ATC programs if you don’t always want to fly on Vatsim.