Heroes 5.5 WarGame Edition - Tactical Edition.

Plague is, as always, the most damaging spell :D

Tier 3 spell that scales up to the level of a top-tier 5 spell with the corresponding perk; that's how it should be, considering its nature (it won't deal all that damage right away).
The same can be said about Tier 3 Righteous Might in Haven – with the perk, it becomes a top-tier light spell for dealing damage (albeit with troops).
And Tier 3 Ring of Deadly Cold will also be quite effective against tier 7.
We haven't looked at summons yet, but there, Mines at tier 4 will also be competitive in terms of damage with Plague, because they also don't deal all the damage at once.