Necropolis and Demon.

It's not that difficult to take a Castle from Daleb, considering his skillful use of machines and the ballista with fire arrows (which essentially ignores half of the armor, meaning even through a wall, the damage will be significant). A catapult has a 40% chance of destroying a wall/gate/tower with a single shot, and if the gate is breached, the siege won't last long. And the gating during the siege will provide decent support.

Well, as far as I understand, Lordesat only plays epic battles where the army is more than twice the normal amount, over 9000. In such battles, Necromancers are naturally unmatched (although a Winged Angel of level 28+ can still be tried, maybe something will come of it). But to consider Necromancers the strongest race just because of that, I think is a misconception. In the early and mid-game, for example, Dark Elves are very strong, and I don't consider them the strongest because of that.