Hm. I'm going to have to go with the 'bit of both' idea. Ziyal's unconditional love for her father, loving him through all he'd done, being a hero to her, had to have influenced him a little. The fact that her death was the event that triggered his descent into madness says to me that he did love her in some way, especially when you consider the substantial loss of status he took upon returning to Cardassian with her. It would have been easy for him to kill her on his way back to Cardassia, claiming that those around him while in Bajoran/Federation space would not have taken well to her death in their territory by his hand, but instead, he accepted her. However, his love for her certainly wasn't as unconditional as her love for him - he pushed her away when she showed an interest in Garak and when she wouldn't return to Cardassia before he announced Cardassia's alliance to the Dominion, but at the same time, he did forgive her once Bajor signed the treaty.
Dukat is an actor, yes, playing the right notes for those around him, but I think that in some way, on some level, he did love Ziyal. I like Deranged Nasat's
view on this, that he was so interested in her being there for him because she had such a pure and selfless love of him, and while he couldn't achieve that himself, he felt as if he could do it through her. I also like the point raised about how he had other children with a wife who, aside from Dukat's statement of how the Maquis will be eliminated by his son's birthday, we don't hear about. He turns all his focus on his illegitimate daughter who has the unconditional love for him rather than the family who left him.