From my own experiences and reading of the views of others, the most difficult Western Philosophers to Read, Grasp and Master are as follow;
- 1. Heidegger
I have covered Kant fully and is now researching into Heidegger in details. I have spent 3 years full time researching Kant's philosophies.
I have only done a quick read of Hegel's books but find it tough going. However I did not go into it in detail and with seriousness because the end result of Hegel is the 'Absolute' which is illusory.
Currently I am reading and researching Heidegger. The toughness of Heidegger is his very extensive paradigm shift from the typical Western Philosophy view of 'being.'
In addition Heidegger introduced tons [much more than any other modern philosophers] new concepts and new meanings to existing concepts, note this;
So far I have spent nearly 3 months full time on Being and Time only and I think I will need many more months before I can grasp and master Being and Time. Thereafter there are another 120 volumes of books [large and small] by Heidegger to cover [other than a few don't think I will read all of the 120]. Besides there are also many secondary sources that need to be read.