Valtor for sure.

Ehhh

I mean, I think the most notable thing that Demande did was help Wise Man turn Chibi-Usa into the Black Lady, but after that, he isn't too notable. He basically goes coo-coo when he realizes that he's now useless and tries killing everyone by bringing the Legendary Silver Crystal of the past and future together, but he's stopped by Pluto and it's after her sacrifice that snaps Chibi back to normal, thus allowing her and Usagi to defeat Demande pretty easily [with their Super Sailor Moon powers].

Valtor was much more calculating and a more hands-on type of person. I know a lot of the villains in Sailor Moon relies on henchmen to do most of the dirty work, but most of the members of Black Moon are forgettable to say the least. Valtor didn't use the Trix that much, but they're much more memorable (to me, at least). He's also stronger than Demande, physically and mentally, as Valtor can only be destroyed by the water stars as the Winx's Enchantix rarely does anything, and he always tricks Bloom into getting moody and cut off from the world just so the dude can escape a less favorable situation. He's more calculating and acquires much more practical magic and objects during his reign, whereas the Black Moon are only focused on getting the silver crystal, and they don't even keep it for that long. Valtor was also the manipulator, whereas Demande and his brother were both manipulated by Wise Man.

^
I finally started to read the Sailor Moon manga(I finished the Dark Kingdom arc and the Black Moon arc) and I have to say that I prefer the Black Moon members much more in the anime.The manga is of course superior to the anime when it comes to the plot(especially Pluto sacrificing herself)but I thing that the anime did better than the manga is that they actually gave some character development to the minor villains.The henchmen of the main villains are quite forgettable in the manga because we don't get to know anything about them,we just see them doing some evil stuff and than being killed by Sailor Moon or by one of the other Sailor Senshi.However in the anime they get much more screen time for exaple:the Ayakashi Sisters who just randomly showed up in the manga,kidnapped one of the Sailor Senshi and than get killed by Sailor Moon have more screen time in the anime,are more sympathetic and are eventually redeemed.
Now lets get back to Demande.I agree that he is pretty much useless.In the anime he at least becomes a somewhat likeable character towards the end when he gives his life to protect Sailor Moon but in the manga he is just insane.

Winx rocks!

Imma go Valtor on this one.