Esmeraude, she's classy, beautiful AND funny. She's not only my favorite Black Clan member but one of my favorite Sailor Moon villains in general.
Others I like: Berthier, Calaveras, Safir, Black Lady (tho I like her better in the manga)
Koan and Petz are ok, I do like Pet's little romance with Safir. Prince Demand is too much of a creep/potential rapist for my tastes. Wiseman/Death Phantom is ok, in his defense, he's probably one of the better "Big Bads" in Sailor Moon, he does at least have more personality and development than, say, Queen Metallia or Master Pharaoh 90. For whatever reason, I've always really hated Rubeus, he's an asshole and his character design is ugly.