Are best-paper awards for conferences rigged/pre-decided? Recently I had an acceptance from a very good conference in our field. Three out of four reviewers gave excellent comments, while one gave a poor comment.
I was telling my friend that if I had had one more good/excellent review, I could be considered for the best-paper award. 

He told me that usually best-paper awards are already fixed and given to the person of the organizer's choice i.e. if the organizers want to invite a distinguished professor, they award their paper etc. etc. So even if I had  had an excellent review from the 4th reviewer, there was no chance of getting the award.

What is the backdoor working of best-paper awards - how are they selected?
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.