Which singlet dihalocarbene has the strongest carbon-halogen bond?
Singlet carbene is zwitteronic, meaning the $\mathrm{sp^2}$ orbital is doubly occupied resulting in a negative $\mathrm{sp^2}$ orbital and a slightly positive empty $\mathrm{p}$ orbital. Therefore it attracts electron on one orbital and repulses them around another orbital. The huge reactivity of the singlet carbene comes from this zwitterionic character.

Below is the picture electrostatic potential of on the left a singlet carbene with doubly occupied sp2 orbital and the right triplet carbene.



It can be argued in many ways how a halogen stabilizes the singlet state of carbene.

Two explanation usually used are:


Electronegative species increase the $\mathrm{s}$ character of σ orbital (Bent's Law).   
Electron donation from the electron pairs into the empty carbene $\mathrm{p}$ orbital.


Though the second one is usually used, I suggest you follow the first (more physical). Adding electron negative substituents increases the s character of the doubly occupied sigma in-plane orbital of carbene stabilizing it, significantly decreasing the energy of the system. Following this concept you can see that flourine should provide the maximum stabilization and so on.

$$\ce{F} &lt; \ce{Cl} &lt; \ce{Br} \ldots$$