water, acids for the sour taste, coloring, caffeine and sweetener that is more effective than sugar and just activates the sweet taste buds, but has no significant nutritiinal value.
How is this the top comment... Sodas are flavor extracts at their core. The amount of other things are just there to balance or optimize those core flavors. Sweet, salty, acidic, viscosity, color, fizz... All choices on top of the flavor extracts. Diet soda just substitutes sugar for low calorie sweeteners.
For which the original answer is insufficient, but I guess my clarification wasn't eli5. The flavoring extracts do contribute calories but they are too low to be reportable by federal standards and many artificial sweeteners also have calories but are also too low to report.