What are the strongest summons?

[Jasper]

Kithironic Lions are much improved from Dom 1. They cost less, while the seasonal spirits cost more.

Pride of Lions is less interesting, but more widely castable. The lions are a bit easily killed for my taste, but I can easily see using them in an early conflict.

Vine Ogres might also be worthwhile, but I'm not sure they're worth the time investment. They'd be much better if you got 3-5 Ogres for 3-5 gems.

The various drakes look ok now that Seasonals are more expensive. The Wyverns look like perhaps the best of this bunch, although I haven't tried them.

5 Gems for 3 Bog Beasts is a good deal -- especially for Miasma C'tis who starts with them!

IMHO the lesser elementals are still decent, just not so obvious a tactic as before.

Earth doesn't really get anything I like until level 5, either Clockwork Horrors or Gargoyles.

Tien Chi's summons would be good if Tien Chi was!