The best hair color for a man is black. Men with black hair look more professional and down to earth. Brown can also work but not the light brown type. The black hair needs not be too dark so as not to look fake.
I'm assuming that you're wondering about brands, and not actual color... There is no actual color which is best, since that is a matter of opinion, not fact. Just For Men haircolor is very effective and inexpensive. It is absurd to say that any one color does or does not look "professional". I've known complete village idiots with black hair, and total geniuses with brown or even blonde hair. Such stereotyping is not only ignorant, it is completely unfounded.