The most challenging campaign.

If we're talking about the Tribes of the East expansion, which I'm currently playing, the most difficult mission is capturing the Holy Flame. You build up an army, the danger level is very low, but a catapult with fire arrows just tears you apart.
I'm playing through it for the second time, and I haven't reached it yet.

Why bother building up a Necropolis army when you can go with magic and destroy all the opposing armies? You can destroy a ballista with a single hero spell if you want; luckily, there's a mentor on the map. In general, this mission can be completed with a single stack of vampires. There's also an option with 14 ghosts, but it's a bit more challenging.
The second Orcish mission, where you have to defend the second city for 6 weeks against armies that are "no match," is definitely more difficult.