Obviously Shadowking if I had to pick one.
Everytime the character appeared it was always sort of a curve ball effect acting wise, story wise, mood wise, visual wise, ect.
David was sort of interesting but did kind of feel stagnant and sort of vacant from a story standpoint at times while the Shadowking felt more spelled out by the end of the season oddly enough than David.
The other characters were impactful and interesting too. The range of acting Audrey gave and the odd visual's associated with the reveal as King and the various monsters definitely brought the Shadowking the most interest I think by far though.