I operate with Win 8.1. My favorite jet to fly is the Eaglesoft Citation X 2.0. I had some issues with win 8.1 blocking the citation from working properly initially, but once you turn off enough of Win 8.1 features, it is awsome to fly. its neat flying on Vatsim and passing the airliners enroute.
the turboprop i fly for more local flights is Areosoft's Twin Otter Extended edition, which also works smoothly with Win 8.1.