For a Design trainee what is the best designing software to learn?

There are few which are easy to learn AutoCAD, UG NX, CATIA, Solidworks

Autocad, solid works, catia

You can use Solidworks or Cartia

I recommend you to use Catia in  this case 

Catia is the best software for designing trainee.

The best design software are Solidworks or AutoCAD

Solid Works Because,

With SolidWorks solutions, you and your team can easily manage product data, share designs, automate workflows, and securely share design data across your organization, with clients, and with external partners. You can deliver compelling product detail with representations of 3D models, 2D drawings, animations