I have enjoyed Promethean starting as Kir'Ko . The combination works very well since you can make your psionic attacks deal thermal damage with focus: flames. There's also promethean mods that do really well with the Kir'Ko units, like the phoenix launcher.
Below are the two Kir'Ko units I ended up using the most:
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Promethean and Kir'Ko is probably something of a left-field combination, why I am happy it worked as well as it did. Promethean seems like a strong secret technology in general, more because of its operations and mods than the units.