Motley trolls everyone. It would be unfair of him to discriminate against someone just because they are a Kabalite! He's very fair.
I'd say Solitaire. Harlequins are the badasses of the Eldar and Solitaires are the badasses of the Harlequins. Would be fighting quite a long while though.
I'm a Dark Eldar fan from the start and as much as I want to say the Succubus then I would be lying. Solitaires are playing the role as She Who Thirsts. The Exarch may be able to have afew lucky parry and blows in the fight but the Succubus would make quick work of her before she experiences the Dance of Death from the Solitaire.
Drazhar would kill the Solitaire because Drazhar, then Drazhar would murder Jain Zar because even Night Lords kill her.