I can normally manage Paul, but I agree that Enigmatist can be difficult at times. I also often have to struggle with Taupi, but I think that the prize for the most fiendish compiler must go to Araucaria.
I have to disagree - Araucaria is the easiest, though he does vary. His clues can be... um unconventional, but rarely impenetratable, and he doesn't that often have obscure words in the solution.
Bunthrone always had very interesting and clever literary allusions; Araucaria has some very good thematic crosswords but I would not describe them as "fiendish"; Paul is very good with long anagrams; overall, I would say Enigmatist is the most difficult, but a lot depends on just hitting the right wavelength or even what mood you are in! I think they are all brilliant.
I agree with dzug & Argee 2 Araucaria has a wide range but i always feel in with chance with him. Bunthorne was one of the most difficult, but although they don't seem to appear to often i always struggled with Fidelio & Fawley. Rufus always seems the easiest.