400-900 nm if possible . They have a good spectral range and the mean path (optical) also would be useful for bio-imaging , considering the interaction of light + material .
Near infrared is good if you do not want to see the light pattern, because the quantum efficiency of the detector (i.e. camera) reduces as you move farther from red.
If you do not mind the light pattern being visible, go with green, since normal cameras have more green sensors than red or blue. (you can also opt for monochrome sensors which have their Bayer filters removed). Also depends on the material you are trying to image and its sensitivity to that chosen spectrum.