Summons to build a strategy around....

[Shovah32]

In the school of conjuration the following are good although many of them shouldnt have a strategy built around them.
Tartarians
Wraithlords
Banelords
'Sleepers'
Abominations
Tarrasques
Naiads(not too powerful but can make clams for a late game boost)
Streams from Hades(gets you a skilled death mage)
Spectre(can give some diversity)
Summon Ghosts cast by a high death mage(D2, 2+ ghosts and 9 gems. Scales nicely)
Lamia Queens(magic Diversity)
Troll Kings(E2 mage with boots of earth can cast to give you an E3 mage, 4 with boots. Also good fighters/thugs)
Sea Trolls(Very tough. Combine with shark attack for a powerful underwater tactic)
Ivy Kings(Similar to troll king magic boosting strategy but with nature. Can get you big vineman/ogre armies)
Faerie Court(allows nature nations to get air magic+healers)

Of course some are stronger than others but those are some which i find very useful.