The way to work it out, is to write out a dissociation equilibrium, and then examine the bronsted lowry conjugate base.
The most stable conjugate base will have the most acidic conjugate acid.
Often, the conjugate base is an anion. The way to work out if an anion is stable is to see what kinds of stabilisation it has (eg mesomeric, induction, electronegativity).
Write out each conjugate anion and the types of stabilisation and we'll take it from there.