What is the composition of diet coke?
The major ingredient in Diet Coke is water (H$_2$O). The second ingredient is carbon dioxide (CO$_2$), about 30 psig, or three times atmospheric pressure. (I measured it with an automobile tire gauge.) Since CO$_2$ is soluble about 1.7 g/Kg at 20$^o$C at atmospheric pressure, Ref 1, this is about 5 g/L, or ~0.5%, using a density for Diet Coke of 1 g/L Ref 2.

So, in the list of ingredients for Diet Coke, Ref 3, everything will be between a maximum of 0.5%, down to about 0.01295% (&quot;46 mg caffeine/12 fl oz&quot; on the label of my 2L bottle).
And, as Matt Black noted, the recipe varies from country to country, and from time to time (and is noticeably different in cans vs fountain delivery, according to those with highly developed preferences), but the big impacts are provided by
#1. water
#2. carbon dioxide
#3. artificial sweetener
#4. caffeine and
#5. flavoring, which might include a little more acid, like citric or phosphoric.
Seltzer is just #1 plus #2; many caffeine-free beverages just leave out #4.
Ref 1. https://www.engineeringtoolbox.com/gases-solubility-water-d_1148.html
Ref 2. http://web.physics.ucsb.edu/~lecturedemonstrations/Composer/Pages/36.34.html
Ref 3. https://www.coca-cola.ca/brands/diet-coke; see also https://www.huffpost.com/entry/coke-zero-sugar-taste-difference_n_597f71d7e4b00bb8ff3875d9 for varieties of Diet Coke