Moving between universities for PhD studies I'm studying for an MSc in Theoretical Physics. I did my undergrad studies at university A. I then moved to another country to do my Masters work at university B.

I did my BA thesis with a big name in the field who gave me great letters of recommendations, and resulted in my being offered a research masters project by another professor at the same university, also a huge name in the field. In the end they both suggested a taught Masters would be a better idea before research. I'm now at B, and it's likely my thesis advisor will end up being an equally big name, with a very long history of collaboration with the professor who offered me the research masters.

I would like to return to university A for my PhD studies. Getting accepted should not be a problem - they know me, I've done good work there, and will have experience with a collaborator. 

However, would it be better for my career to perhaps go to university C, for example, which is also full of collaborators of everyone previously mentioned, for the sake of more experience? 

Essentially, would returning to university A be likely to have a detrimental impact on future employablity; getting accepted to postdocs, etc? Or would letters of recommendations be most important and my research output?
It seems that you've got lots of great options available to you - kudos to you!  Given the situation you described, I don't think returning to university A will likely have a detrimental impact on your future.  If your work is good and you get several papers published at competitive journals/conferences/etc., that should matter much more.

The exception here might be if university C is considered far more rigorous than university A.  For example, switching from a state school to MIT or CalTech (though even this isn't a perfect analogy - it's largely field-dependent).  Outside of that though, the university itself will likely matter less than the research you're doing.

And as always - follow the funding :)