What is the conjugate of I?

The conjugate acid of ${I}^{-}$ is $H I$, hydroiodic acid.
Note that when you speak of conjugate acids/bases etc. you must be mindful of charge as well as mass. If we remove a (formally) proton from $H I$ we remove ${H}^{+}$ we remove charge as well as mass, hence the species left behind is ${I}^{-}$.
What are the conjugate acids of ${F}^{-}$, $S {O}_{4}^{2 -}$, and $N {O}_{3}^{-}$. Which is the strongest conjugate acid?