Which cad software is most frequently used worldwide ?

It all depends in which sector you are working...

Aerospace - CATIA, SolidWorks ... some AutoCAD

Automotive - CATIA, SolidWorks, Inventor ... old files in AutoCAD

Government and large Pharma ... Micostation ... old files in AutoCAD... Arcinfo 

small businesses and architectural ...Solid works, AutoCAD... what ever they can afford

So what is the need for your industry ... I have seen AutoCAD going out being replaced by Inventor or SolidWorks ...

The big companies usually stay with CATIA or Microstation ... at least that is what I have seen

In general if you have the proficiency of one CAD program ... you have no hard time learning an other 

switching from drafting2D to modeling in3D is not hard either .... you just have to have the concept down understanding what you are drawing in your field .... you might have a problem being in civil engineering and trying yourself in mechanical engineering unless you know how things interact in a machine.

You have a lesser problem if you come from the mechanical field and go into Industrial Planning or Civil Engineering ... at least I did not have a problem

To answer you question direct ... In my opinion I think AutoCAD is still the most frequently used CAD program ... I already outlined the reasons above. 

All the files we get in from Architect's and Mechanical Engineers are Autocad and in3d. All of the large jobs are requireing Building Information Modeling, which is a fancy term for smart3d objects.

We use Autocad MEP and a3d piping add on to produce the mechanical systems inside of a building before a construction worker sets foot on site. To say autocad doesnt do3d is not accurate, it depends on what you want out of it.

Autocad is the clear leader in the construction industry, and is leading the way in BIM with their revvit product.

So as miss.Elke was saying, there is no in general "best product", however there are best products for specific applications. If you tell us what you want to accomplish it will help us to recoment the best software.

AutoCAD and AutoCAD LT without a doubt, if you talk about market share worldwide. If you want to go in details by looking into individual industries or specific countries the situation is very different case by case.

The last independent report I have read (released by JPR in April this year) was crediting Autodesk with almost30% of the market followed by Dassault with about22%.

Auto Cad is mostly used but i think in future BIM will replace it totally but it will take sometime .

......................I think Autocad2d

AUTO CAD is most frequently used software and  PRO-E is also famous .

Auto CAD is the most commonly used software for computer aided drafting. 

Autocad- which is most commonly used in worldwide

Its Autodesk Autocad without any doubt. 

For CAD - AutoCAD is on top for CAD/CAM both Solidedge and Pro E are on Top