I have a question for the Google Adsense user, what is there high income from one click with one keyword.
Loan, credit, banking and financial services, etc. I wouldn't go with these niche tho, they're highly competitive.
Adwords tool doesn't show avg click values anymore, does it ? @ OP : Are you asking about highest paying niches or Highest you can get per click for any keyword ?
Try http://www.spyfu.com/ and http://www.semrush.com/ to find out targeted keyword's CPC. Of course the information is approximative and sometimes inaccurate, however that's better than nothing.
Keep Slow and seedy going by Ad scene.Because which best in this world they must follow the rules & regulation.