Selection of wavelength for optical reflective sensing
You can look at the output (when converted into some common units) for various LEDs or lasers and compare the sensitivity of various sensors. 

Optimum is probably IR as used for TV, A/C etc. remote controls and optoisolators but I don't think there is a huge difference (especially with highly optimized modern visible light LEDs) unless you get to very short or long wavelengths such as blue or UV and far-IR.  

The reflectivity of the target probably makes a much bigger difference. Retro-reflective targets such as the material used in highway signs or corner reflectors  have been shown to work well in practice with distances up to about \$3.8\times10^8\$ meters.