Well... it is well known that "mesothelioma" related sites can generate high adsense revenue...so does real estate & forex as cohenpapar pointed out and many others... the problem here is that those niches are so competitive you would have to do some SERIOUS keyword research and to find a keyword you have a chance of ranking.. not to mention you'll have to drop big money backlinking your sites.....
Agree Real Estate, Forex are the ones that have worked for me. The highest paying CPC, you can also give a try to jewellery related.