Do you know what are the top 20 highest paid adsense keywords? Checkout this infographic
Bear in mind, don’t start targeting above keywords if you run adsense sites, they are impossible to rank on organic also very expensive when it comes down to paid advertising. What you can do instead is to take the keywords and start digging into their sub niches i.e. divorce lawyers in London, Hair Rehab London etc…
Long tail is your best bet, try to get into sub niche and most probably you would make some money via Adsense.