Dr pepper or mountain dew
Which soda has the most fizz?
Dr. Pepper.
While looking for a good link I came across this one---Not on point, but definitely answered something I've always wondered about.
As to your question "specifically," here's a "general" answer. (Sorry about that.)
The Solubility of carbon dioxide in the basic solution that composes the drink will determine how much is dissolved in a specific drink.
And also, the different pressures and temperatures will affect the rate at which the carbon dioxide comes out of solution thereby giving the illusion of different "fizziness" of a given drink.