Which file format is considered the most universal?
Most Universal for what purpose?
Can you tell us why you want to know this; what are the type of things you need to convey in your files?
There is no definitive answer, but you can get some good feedback if you tell more about your typical workflow and the type of applications your files have.
As an example, when you need to do 3D printing with objects that are scanned, I would suggest to stick with STL files. However if you are making parts to be milled from aluminium, I’d recommend STEP files. If you are creating 2D drafts you are probably best off with DWG or DXF.
All depends on workflow and purpose.
Sorry I should have been more specific. It is basically a model file that is to be added to an assembly drawing. I don’t foresee any need for 3D printing.
Am I correct in assuming by assembly, you mean an assembly in SolidWorks?
If so, I think you best go with the STEP format.