Why is chlorine and not fluorine labelled as the most reactive halogen for halogenation?
The reactivity of halogens in sunlight 


OK, this tells us that we are dealing with a free radical reaction.  Sunlight breaks the halogen bond ($\ce{X-X}$) to generate halogen free radicals.

$$\ce{X2 -&gt;C[{h\nu}]\  2X.}$$

In the next step the halogen radical abstracts a hydrogen atom from any hydrocarbon present to generate $\ce{HX}$ and a hydrocarbon free radical (note that multiple hydrocarbon radicals may be generated depending on the structure of our hydrocarbon).

$$\ce{X. + R-H -&gt; HX + R.}$$

Finally, the hydrocarbon radical can react with the halogen free radical to produce a halogenated product.

$$\ce{R. + X. -&gt; R-X}$$

Let's now consider the energetics of these steps.  The first step, generation of the halogen free radical, uses the sunlight to break the $\ce{X-X}$ bond, so no additional energy is required for this step. Then we break a $\ce{C-H}$ bond (so we must add energy) and make an $\ce{H-X}$ bond and a $\ce{C-X}$ bond (so energy is given off).  The various bond strengths are contained in the following Table.

\begin{array}{|c|c|c|c|c|c|} \hline
\ce{C-H}~ \text{Bond Strength} &amp; \ce{H-X} &amp; \text{Bond Strength}&amp; \ce{C-X} &amp; \text{Bond Strength} &amp; \text{Overall}  \\ \text{(kcal/mol)}\ &amp; &amp; \text{(kcal/mol)} &amp; &amp; \text{(kcal/mol)} &amp; \text{(kcal/mol)} \\ \hline
\ 99 &amp; \ce{H-F} &amp; -135 &amp; \ce{C-F} &amp; -116 &amp; -152\\ \hline
\ 99 &amp; \ce{H-Cl} &amp; -103 &amp; \ce{C-Cl} &amp; -81 &amp; -85\\ \hline
\ 99 &amp; \ce{H-Br} &amp; -88 &amp; \ce{C-Br} &amp; -68 &amp; -57\\ \hline
\ 99 &amp; \ce{H-I} &amp; -71 &amp; \ce{C-I} &amp; -51 &amp; -23\\ \hline
\end{array}

(bond strengths taken from Bill Reusch's table)

We can clearly see that all of these free radical reactions will occur (e.g. they are exothermic) in the order you suggested, with fluorine being, by far, the most reactive.

$$\ce{F2 &gt;&gt; Cl2 &gt; Br2 &gt; I2}$$

Saying that chlorine is the most reactive is incorrect.

Note: 99 kcal/mol was used as a typical $\ce{C-H}$ bond strength.  We could refine our model by using more exact $\ce{C-H}$ bond strengths (see this Wikipedia table) and calculate the relative rates of halogenation at the various carbon atoms in a complex hydrocarbon.