The card rank is calculated by summing the cards Red Attack power, Green Attack power and Yellow Attack power. In instances where two cards have the same overall power rating then they are sorted in alphabetical order.
This is by no means a perfect way of determining the best or worst cards in the game and doesn't take into account power option support, special effects, Digivolution lines or anything else. However this does give you a high level overview of what cards are likely to be better than others.
You can view a complete ranking list of all Digimon on the All Rankings page.
Here are the top 10 strongest cards of each level: