It depends on what you want from the sim...
If you want every airport, ILS, VOR and GPS approach in the world, and want to fly through every mountain range in the world with top class scenery etc., and want the most advanced multiplayer environment in the world (that even the military use), then go with Microsoft Flight Sim 10 or FSX for short. In short if you have a good PC (good video card particularly), then the latest version of FSX rocks. The missions are fantastic and allow you to live out your dreams of dropping aid in Africa in a DC3 after which you loose an engine over the mountains and have to crawl back to a dirt strip. There are a ton of them and they are all great. I particularly love the one that puts you flying the 747 to minimums during a major storm in Asia somewhere. There are a ton of addon's for FSX also and I believe that includes RV's
I personally don't like the feel of XPLANE or how realistic it feels compared to FSX, but each to their own.
Also FSX is only for windows so if you have a Mac you are out of luck.
H.