What is the most polar covalent bond?
There are various electronegativity scales. 

Electronegativity vales from Wikipedia

$$\newcommand{\d}[2]{#1.&amp;\hspace{-1em}#2}
\begin{array}{lrl}
\hline
\text{Element} &amp; \text{Pauling} &amp; \text{Allen}&amp; \\
\hline
\text{H}  &amp; 2.20  &amp; 2.30 \\
\text{C}  &amp; 2.55  &amp; 2.544 \\
\text{N}  &amp; 3.04  &amp; 3.066 \\
\text{O}  &amp; 3.44  &amp; 3.610 \\
\text{F}  &amp; 3.98  &amp; 4.193 \\
\text{P}  &amp; 2.19 &amp; 2.253 \\
\text{S}  &amp; 2.58 &amp; 2.589\\
\text{Cl} &amp; 3.16 &amp; 2.896 \\
\hline
\end{array}
$$

The compounds are all binaries, so the polarity, $\mu$, is equal to the difference in charge, $\delta$, times the distance, $d$, between ions ($\mu = \delta \cdot d$). Only given a table of electronegativities, the first level of analysis would be to assume that all the bond lengths were equal and that that the polarity would simply be a function of the difference in electronegativity. The differences are thus:

$$\newcommand{\d}[2]{#1.&amp;\hspace{-1em}#2}
\begin{array}{lrl}
\hline
\text{Element} &amp; \text{Pauling} &amp; \text{Allen}&amp; \\
\hline
\Delta\text{Cl-F}  &amp; 0.82  &amp; 1.297 \\
\Delta\text{S-O}  &amp; 0.86  &amp; 1.021 \\
\Delta\text{P-N}  &amp; 0.85  &amp; 0.813 \\
\Delta\text{C-Cl}  &amp; 0.61  &amp; 0.352 \\
\hline
\end{array}
$$

So on the Pauling scale S-O should be the most polar, but on the Allen scale Cl-F should be the most polar.