The cost-per-click - how much you would pay for a click if you were using AdWords to rank for a keyword - is a good indication of how difficult it would be to rank for that keyword organically. In theory then, this list of the 100 most expensive words will closely resemble the 100 most difficult words to rank for organically.
Infographic by: semrush & webpagefx
Infographic by: semrush & webpagefx