For the first question, we want to look at potential intermolecular forces that are acting to hold the molecules together. The one with the weakest IMF will have the lowest boiling point.
CH4 has only dispersion forces while all the others have dispersion PLUS either dipole-dipole (HCl, H2S, NH3), and/or hydrogen bonding (NH3). So, CH4 will have the lowest boiling point.
For the second question, we want to look for the molecule that dissociates/ionizes into the most number of ions. This will be NaCl(l) since solids don't do so and H2O(g) only ionizes ever so slightly.
So, the answer is NaCl(l) is the best conductor.