The kind that you can show the buyer that it values the amount you're selling them for. Do a niche you're familiar with and good at. Something that you can make successful. Once you can show potential buyers that the site you created can provide them with revenue, and your price is right, then you'll sell it quick. Hope that helps.
The more current and trendy the topic/niche is the better most likely. If you can build sites in certain niches that are on the rise in popularity yet at the same time not overly saturated then your chances of selling should be very good.
another good niche is video, with movies/anime that you can watch. you can also make money from ads on the site etc
Making money and investing has some good keywords in it for Google Adsense to put up relevant ads. I have seen clicks over a £
Can't say for sure, it all depends on how's browsing the thread looking for sites when you post your site....different things sell at different times....weight loss is always good, health, pets, etc.
A high traffic blog goes to higher rate always ! that's the general fact every where. A niche which is always growing will have higher demand...