[SlipperyJim]

I'm not sure that you really want to build a strategy around any single spell, even the really cool ones. There's simply too many wild possibilities for what can happen.

Still, the Conjuration summons that have already been mentioned are very good. I'm especially fond of Ghosts because of the way that the efficiency of the spell "ramps up" with more levels in Death magic. If you can get to D3 (maybe D2, with Forge of the Ancients active), you can "bootstrap" your way into higher levels of Death magic with the right booster items. So now you have a couple of D4 (or higher) mages. Let those guys spend a few months summoning Ghosts, and you have a very respectable force.

Of course, the Ghosts themselves are pretty good. Stealthy, ethereal, life-draining, fear-causing, chill-bringing ... what's not to like? Their only weakness is that a Ghost's only real protection is being ethereal, so magical weapons and spells will shred them quickly.

Another great Conjuration spell is Earth Attack. No, it's not exactly a summoning spell, but it's in the same school of magic. Earth Attack is an assassination spell, arguably the best of them all. Any E4 mage (see Shovah32's note about the Troll Kings!) can send a monstrous Earth Elemental to kill nearly any commander.

Finally, don't forget the Elemental royalty. As unique summons, it can be hard to form an entire strategy around them, but they're certainly powerful enough to be a part of your strategy.