Best for whatever reason. So your parameters can be, for some examples, that your pick has more good entries than any other franchise, or the highest percentage of good ones, or the series as a whole averages out to be better than anything else, or maybe all together your choice has the most compelling concept, or it changed movies more than any other franchise.
Or maybe you just simply enjoy them the most!
My pick is the Back to the Future trilogy. I love all three films pretty much equally, they set out to tell one over arching story and achieved that, and it isn't pulled down by 5 other terrible sequels. It ended with dignity and to me, it is the only perfect franchise.