It has to be ghost riders for me. There's nothing that compares to the feeling of taking out several enemy provinces each turn without even having to move out of your cozy castles. One casting is enough to take out any decent amount of PD unless it is equiped with magical weapons. If your oponent tries to retake your newly acquired provinces using small forces of his own you destroy them with two castings. If he sends out a reasonable force or thug, you can teleport something really big and nasty on top of him. This spell helps tremendously to take and hold the initiative in the decisive lategame wars. (coupled with more traditional raiding tactics. )
Of course, if you don't have native D4 mages it does require a rather serious investment to get yourself a stable of casters ready. (Lich+skull staff+skull helmet = 54D with a hammer. If you do have D3 mages you can give them the hardly inexpensive RoS for the same effect) On the bright side, these are the same guys you want to summon your tartarians (with a RoS on top of the above equipment) so you'll want to get as many as you can afford anyway. They can chain-summon Tartarians for you to Gift of Reason the unafflicted ones until the war begins, and then seamlessly switch to Ghost riders. My current game is the first time I didn't manage to get a nice number of casters ready before end game.
Of course, this time I have access to send horror casters, so I can hopefully accomplish the same thing.