In Archicad, what is more efficient, making 3d design directly in 3D navigation or make 2d of every floor than see the results in 3d or finally work in parallel way with 3d-2d-section-Facade ? & Why?

First step to start your design with2D basic drawings. Then it will be more efficient to modify your design in3D navigation to see the real effect in your model.And to get better understanding visually of your design.

This is the main advantage of using Archicad. Because it work as BIM applications that can help you to generate all drawings in fast time. In Archicad, you can make modification in any view and automatically you will find it modified in the other views.

And what about the best rendering programme for archicad ?

With the implementation of CineRender engine by Maxon in ArchiCAD a high performance CPU becomes even more essential than before. Besides rendering, the industry-first Background processing (available from version) uses multiple cores intensively. Background processing greatly helps the performance, navigation and display of extra-large, complex models