[albinoloverats]

It doesn't say exactly where the data comes from but:

"Passports are ranked based on their Visa Free Score. The higher the Visa Free Score, the better the Passport Power Rank."

Edit: I presume this is the number of countries which a holder of a certain passport can visit without needing a visa.

[erroneousfunk]

I think it would be more interesting to see this ranking weighted by the "desirability" or size of the countries you can visit, or some other useful metric. If I can visit 177 countries, but those 177 are "all of the countries except the ones in the EU" my passport might have far and away the highest power rank, but I certainly wouldn't want it.

In this case, the power ranking might involve some sort of Google search type of algorithm, where the "desirability" of the country is weighted by how many countries its passport holders can visit (we're assuming that the countries enter these sorts of visitation agreements based on political stability, trade agreements, tourism motivations, etc)

In addition, you'd probably want to factor in a more detailed breakdown of visa restrictions, difficulties to obtain visas to various countries, etc.