Unity is a multiplatform game development tool, designed from the start to ease creation. A fully integrated professional application, Unity just happens to contain the most powerful engine this side of a million dollars.
The only reason I am not giving a 10 is that the 2d still sucks. Duality engine seems to understand how to properly make a 2D C# engine. However for the 3D side of things, Unity is still one of the best engines out there.