No. Conference best papers awards are not rigged. Often, associate or track chairs will make decisions based largely (but not entirely) on ratings from peer review. Sometimes, there are awards committees who may take recommendations from reviewers or chairs and make their own decisions.
Because paper quality can never been measured objectively, these decisions are subjective. The reflect the opinions of the award committee or chairs/organizers, and peer reviewers as to what the "best" work is. The process is no more rigged (and no less, I suppose) than the peer review process itself which is also inherently subjective.
There might be some very low status conferences where things are truly rigged but I'm sure these are rare and unlikely to be places you should be submitting anyway.