In my opinion I think Mario Party 2 is the hardest because items were too overpriced ,it had some tricky minigames,and Mario Mechs is the hardest minigame in the mini game coaster in MP HISTORY!
Now to think of it Mario Party seems the hardest,I have been recently playing all 3 to compare them and Mario Party 1 is the hardest mostly due by no items and tricky maps like Wario's Battle Canyon (The hardest board in MP history T_T )
I would say Mario Party. The rotating minigames really gave my hands blisters. (I read the newsletter and LOVED IT!) Also, the minigames would be tough and you'd lose coins if you lost certain types of them.