There are very few pictures that look better in black and white. I don't know if it's purely a matter of opinion or not, but most photos just look better in color. I can't tell you specifically what type of image looks better desaturated. Perhaps only images that are so tranquil that having loud colors would distract from the feeling? I don't know.
In any case, here's one of the only pictures I've ever taken that I preferred in black and white.