Your poll quite shockingly did not include Total Chaos AGA which has adjustable difficulty level and therefore IS the hardest fantasy strategy amiga game of all time.
You can simply beef up the settings, it only takes a few secs, then the game is nearly impossible to win. But if persevere you may triumph in the end if you are an expert Wizard such as I.
Dune 2 is a fab game but it isn't even in the genre you stated which is Fantasy. Everyone and their dog Fido knows that Dune 2 is Sci-Fi. BTW I didn't think it was very hard at all except for the last mission. I mean I used to win some of the beginning and middle levels with nothing more than a Harvester; the AI was THAT dumm.