The most challenging campaign.

In general, it's a matter of personal preference. One of my friends easily completed the 3rd mission of the 2nd campaign in version 1.5, which many consider difficult, but he can't seem to pass the next mission on Recruit difficulty. Moreover, he can't capture the last town at the very end of the mission. Soon, I will also find out how difficult this mission is in 1.5 on the Hero class (I previously completed it in 1.41 on the Warrior class) - I just need to complete the 3rd mission.

Another friend easily completed the "Emerald" mission, but has been struggling with the next one for a week (he took a break) - he gets killed quickly at the very beginning.