Are best-paper awards for conferences rigged/pre-decided?
I would find it very unusual and very disturbing if that happened. 

In my experience (Computer Science and related fields), conference committees work hard with submissions to select the best paper. And they debate it, as different committee members will typically "champion" a paper that they think is worthy. Often this happens because the committee member is a specialist and recognizes that some paper makes a really significant contribution to some aspect within their speciality. 

But I've never seen anything such as you suggest, nor any personal favoritism. 

If this sort of thing really exists within a field, I think that it would be a good idea to raise the idea as an ethical issue with that community generally.