I doubt people will tell you as if they were any good they would be using them themselves. Anyway this is simple, just pick a niche - all niches are profitable. Pick something you already know a bit about and that you know isn't saturated. Examples: Bonsai Trees, Ice Skates, Some new up and coming celebrity.
Anime streaming video sites. lol jk. I think over the course of the last 4-5 months, a ton of people have jumped on the anime streaming site bandwagon. So thats a no go.
Yah, do your own niche research.... like someone said, pick a topic you are interested and knowledgeable in so you can create content for it.
Pick a ninche. If you are doing adsense search over the internet to find high paying keyword and occupate in that ninche. If other i do not think it really matter. Whatever niche you pick important is to have original and quality content that will attract more visitors.
My opinion is similar to what others have already said; Money will only come if you know what on Earth you're talking about. Sure, doing a site talking about how to make a million dollars in a week through investing (though very do-able if you have the start-out cash) will lead to a lot of traffic...if the content is good. For content to be good, however, the posts need to be well-written, passionate, but most importantly, knowledgeable. If you know a lot about making money online, then hell write a blog about making money online. If you know about sports, make a website about sports. Do something you love, and it won't feel like work - the traffic and money is just a nice added bonus. For example, my friends and I are writing about deals and reviews about online services, since that's what we greatly enjoy doing! Soon, we're going to try to expand traffic and make some money - but as of now, we're just posting and having a great time, while still getting around 60 Uniques a day (not much but hey...) Feel free to check us out at the link in my signature
MMA is a fast growing sport, might want to jump on that train if it's something that you're interested in.
or do some research on going green. this is something that will be growing (pardon the pun) for years to come. But as said by some others pick something you are interested in.
Somewhat of a loaded question considering if we new the answer to this question, why wouldn't we already have a landing page & ebook on the market making us m!ll!on$...
make sites about things you love, like have interests in. i found if you make a site for money it wont happen... ie setting up a DUI lawyers page, loans page ect ect..they all never get traffic. set up something you love write about it a few times a week and the money will come.
Environment, Sustainability and being green are all particular interesting and growing niches that are having lots and lots of money invested in..