I think that Forge of the Ancients is really cool, primarily because it gives your Forgers a +1 boost in every magic path, even the paths they have 0 skill in. That really opens up a lot of forging possibilities.
Burden of Time is a fun one to cast as Ermor. Of course your opponents will pool their resources and get rid of it within a few turns. But I like to imagine their reaction when they see that you've cast it.
Some day I aspire to cast Utterdark with a ton of extra gems on top of it to keep it from being dispelled.