[Baalz]

Winter wolves and summer lions can be worthwhile sometimes for their cold/heat auras, and I've seen fall bears/spring hawks pull their weight in some situations.

Storm demons and to a lesser extent demon knights are effective in small enough numbers to make it worthwhile to summon them one at a time.

Several summons can be real tide turners for early fights if you buff them. Body ethereal, luck, iron warriors, quickness, regenerataion, etc. and some otherwise fairly useless guys become things that really require a specific counter to keep from dominating the early battlefield with just one or two of them. For instance:
Behemoths
Kraken
Sea serpents (mostly for the fear aura)
Dark vines
Sea trolls
Crushers