I do not think I'm ready yet to the hardest of them all , i am just asking from intrest. Is it just a hard version of solo conquest? Or is it Revenge Of The Dragon Lords(Solo scenario book). Maybe its vullkars return? I DO NOT KNOW
Especially if you include non-official scenarios. I hold nothing against them, and I enjoyed playing many, but then it is easy to create a ridiculously hard scenario which would then earn the title of "hardest of them all".
Amongst the official ones, I'd give the palm to Volkare's quest.
First, lemme assume you are talking about solo/coop, because it's pointless to discuss difficulties of competitive games, in which you try to win your opponent(s) instead of the scenario objective.
Solo Conquest and Volkare quests can be easily adjusted so their difficulties vary according to the setting(s) you choose.
Mine Liberation solo is so hard that someone came up with tweaked solo version. IMO it's harder than the Volkare scenarios with the highest difficulties - which in turn are roughly comparable to Solo Conquest with 18~20 City levels.
MK:SoT scenarios, IMO, is slightly easier than above.