Hi! No body say what's the best niche because every keyword plays different role. Like in UK finance related keywords give you handsome money but the same keywords plays different role in different country. So, 1st you have to decide the regional then go for keyword.
The better question to ask is what are the best paying keywords? Usually in every niche, there are a few keywords that pay better than most keywords. Use the Google keyword tool and select the option to have it show you the cost per click for each keyword. Then I would just start selecting good keywords with a high cost per click.