Simplest software for making 2D and 3D visualizations(animation),

Some years ago Auto Cad had a very powerful but intuitive module calles 3D studio.

It could read as input Acad mesh files and also had its own simplified interface.

In a couple of hours, I could make richly rendered walkthrough clips of projects.

These days there are numerous apps, many are freeware. Here is a link to some. freeware modeling

You can use Keyshot or SolidWorks Visualize, or even maybe Nvidia Iray. Don't expect every easy-to-use rendering and animation software to be free though. If you are looking for a user-friendly rendering process, then I recommend Keyshot. KeyShot has a comprehensive library of physically accurate materials that are assigned to the 3D file by drag-and-drop.

It features real-time rendering process. Whenever you update the environment with auxiliary elements such as animations, camera angles, lights, etc. the software updates the visuals.

For examples on what you can do with Keyshot, you can check out Dan Cabral's awesome animation.