What CAD file format is the right one?

From the user's point of view, the right format is the one you use in your CAD. And as the most used CAD application in the world is AutoCAD - and as most other CAD applications can read DWG - the right format for you is probably DWG. The textual version of Autodesk's DWG file format is DXF - and this well documented format is readable for virtually any CAD software.

For 3D CAD data there is no single prevailing format. Of course, DWG often carries 3D data; in mechanical industry, the most widely used 3D software is Inventor with its IPT/IAM format, there are libraries (entourage, gaming models, cars...) based on 3D Studio old 3DS format or new 3ds max format MAX. But the safest way for sharing 3D data is one of the neutral data exchange formats - STEP or IGES. For BIM data in AEC industry you can find many sources of RFA (Revit family) block libraries.

If you have found a block in a different format than that of your CAD application, you can still use some conversion tool to convert the CAD data to the requested format. The safest way in this case is to use an import (or conversion) function directly in your CAD application, or to use the original CAD application to re-save the data to a neutral format (e.g. Revit to open a RFA file and save it to DWG). There are also standalone general CAD conversion tools and web services but the results of such conversions are not always perfect.

An incompatible version of DWG file

One of the most frequently asked questions when using a Block library with DWG files is how to open a newer-version DWG file format in my old version of AutoCAD?

The AutoCAD file format - DWG - is updated in every 3rd release of AutoCAD. So published DWG blocks saved in a newer version might not be readable in an old AutoCAD version. E.g. AutoCAD 2006 cannot open drawings from AutoCAD 2009.

But the solution is simple, you don't need to upgrade your AutoCAD immediately - just install the free tool - Autodesk DWG TrueView (2008 or higher), don't get confused by its name (it can really convert, not only view) and use its conversion functionality to save back the complaining DWG block to any older DWG format. There is no other option for this - various "DWG conversion" utilities do not save the genuine DWG format and the resulting DWG files can cause many problems in AutoCAD.

See the tips:
How to convert an AutoCAD DWG drawing to an older file format?
Error message: File was created by an incompatible version of AutoCAD
and
www.autodesk.com/dwgtrueview-download

Free web resources of CAD blocks

There are many sites on the Internet which offer free download of CAD blocks and symbols. Some of them are well organized, some are just a heap of DWG files, some have only simple 2D symbols, some are specialized on 3D models, paramtric libraries or AutoCAD dynamic blocks. Some are specialized to architecture, HVAC or electrical, some have mapping symbols, some have only mechanical libraries and standard parts. Even more and more manufacturers offer free CAD blocks of their products on their web sites.

Such web libraries can help CAD users to add quickly the required product, entourage or symbol to their drawings and 3D CAD models.

If you know or run a usefull free site containing blocks and families in DWG, DXF, RFA, IPT or SAT format, let me know - so I can add the link to your site.