Honestly, I don't like the chaos lord. He's too fragile, unless he's the demon prince or bloodthirster, of course.
I'd have to go with the chaos lord. He's not the best support unit. The canoness has better abilities despite her fragile state and gets a good ranged attack.
Maybe the warboss actually. How often do you even see the most lazy primary commander? Big mek is more useful than him...