FSX, Otbiter 2010, maybe Prepar3D in the future. My son is a Kerble fanatic...
Finally built a new system that could run FSX, belong to a very old VA (Lynx). I hadn't flown a real plane since the 70s, and I'd swear that all the simming kept me "current" to the point that I was handed the keys after 2.1 hrs dual (after about 21 years non-flying)...I opted for another couple hrs dual to get the "comfort factor" back up...
I remember back during a shuttle mission, I downloaded the current orbital numbers (the guys at the Orbiter site updated them a lot), and was looking down at the Earth in Orbiter, with NASA TV going on TV. They had the "look down" view selected during the sleep period, andm aside from a few second delay on the Orbiter display, the views were virtually identical...it was astounding to watch.
Lots and lots of addons, their SpaceX Dragon is great fun. Wish some of the deniers could play with Orbiter, just to see how orbital mechanics work. Great Apollo addons available as well...