The following ratio is generally constant for a given species :-

Chargaff's rules for double stranded DNA: A = T and G = C.
From these, purines = pyrimidines: $A + G = C + T$. So $\dfrac{A + G}{C + T} = 1$, and similarly $\dfrac{T + C}{G + A} = 1$ and $\dfrac{A + C}{T + G} = 1$. These ratios are 1 for every organism, so they do not distinguish species.
However, the ratio $\dfrac{G + C}{A + T}$ (or $\dfrac{A + T}{G + C}$) is different for different species but remains the same (constant) for all members of a given species. It is characteristic of the species.
So the ratio that is generally constant for a given species is $\dfrac{G + C}{A + T}$.