Any blog which generates traffic. A site that displays ads that pay $10 a click and only gets 1 click a month isn't going to make as much as a site that pays 10c a click and gets 1000 clicks a month.
Why not look in Adwords to see which words cost the most. Then you'll get an idea of the biggest earners as well except they will also be the most competitive and difficult to beat the big boys.
Make the blog ccording to your interest areas.Only then you will be able to add more and interesting content on a regular basis.Surely some niches draw more traffic.But on web there are people with every kind of interests.So choose the best field and be regular with blogging.
Unique content makes money. It has visitors, they link to the site, the site grows and then you also earn something. if you don't have anything to say in a niche, no use getting it
Dear Studee,
Don't think about High paying keyword to your blog. Write a blog about your interest field. Then you can make little money with high self satisfaction.