What are the strongest summons?

[Taqwus]

For real gratuitousness, wish for a (stealthy) Lord of the Night, give him a Black Heart and some better gear, and now you have a size-6 flying assassin.
As for Gift of Reason... interestingly, when you get a Vastness and GoR it, you find out that it's an Astral-3 mage (good AND bad; bad if you're planning to use it against nations with lots of astral power...).
In terms of building a front line, I like the Wight summons; you get 20 well-armored non-fatiguing non-eating skilled undead warriors for 30 gems. Enliven Statues can also build a decent front line.