[hnchrist3]

To any question that starts with "What is best..." in DOM3 there is only one answer: 'It depends.' Everything is interdependent and based on an overall strategy.

Not to sound trite, but the "real" answer to your question is LA Ermor, as all of its (national) players are summons. Your only (national) commanders are freespawn or summoned. (You may still muster regular units, but they have upkeep costs--yeccch!)

However, even Tartarians are available to anyone if they have the appropriate Death-level and Conjuration-9 research.
[Hint on Tartarians. They come to you powerful but useless because they are feeble minded. Acquire the Chalice (Construction-8; unique) and all that goes away. NOW, you have a fighting force.]

Again, it all depends. If you are a powerful Blood nation, you may use vampires; Seraphs seem popular as well.

A summons in itself is not useful; it must be tailored to either the threat or the conquering strategy.