There are many published rankings of franchise companies. Each evaluates companies using whatever unique characteristics they feel are important based on the type of comparison they wish to make. Given this, the first thing you need to do is look at each ranking to see what they are focused on and how they weigh the importance of each criterion they use for evaluation.
Factors that are commonly used are total size in assets, sales or units. In this regard, bigger is usually rated higher than smaller. Other rankings feature things like rate of growth, market penetration or franchisee addition. In this regard, faster is usually rated higher than slower.
It is probably obvious that just because a company is bigger or growing faster than its competitors, that doesn't mean it is the better opportunity for you. You still need to research the companies based on the goals you are trying to achieve in your life to find the one that's best for you.
The major ranking studies are very careful to clearly identify the criteria they use so that you can figure out whether or not that particular ranking is relevant to the decision you have to make. Take the time to read the forward and figure out what is being examined in any ranking you review.