In term of niche, on the Internet do not have something called best or bad niche, it is depend of the service provided,and the customers demande.
Many of niches are going down, not because no customers but can not reach them, I mean should reach and find customers that love your product,
for directories I think if with specific topic will success for example can be directory of worldwide cooking (I am thinking of it ) or can be for worldwide hotels, I came across one website that the owner is selling dogs'names Amazing. her website is in the main page of google adsense