I suppose it depends where you are in the game. I've just finished Chapter 3 but have not yet gone on to the next as I'm finishing off a few quests. My fav summoning is the Fire Elemental (druid version) - it seems to be the best tank amongst the cannon fodder.
I have the horn (upgraded to silver) and the spider figurine but both those summons don't seem to do well at melee.
Spider spawn (mage spell) appears good, but I've only used it once so I'll need a bit more experimentation with it before I make up my mind.
Of course, insect plague is useful for distracting spell casters.
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"Butt-kicking for goodness!" - Minsc
"Cities always teem with evil and decay. Let's give it a good shake and SEE WHAT FALLS OUT!!" - Minsc
