Depends on the spec of your machine. MS can be a real resource hog.
The two main ones in my opinion are MS Flight sim and X-Plane. For all you can get out of a sim they're as good as each other. Get some good add-ons and you can have some fun.
X-plane is better for overall flight dynamics due to it's method of developing the flight model. I find there is a better feel for speed in X plane as well. It's not as user friendly as MS though. I use MS more frequently.