Here is my ranking on where you should kill them for maximum experience an hour.  Taverley Dungeon - This is the best place of all, there are 24 Black demon spawns, and you can use a cannon and safespot them.  Brimhaven Dungeon - a great second if all other worlds are crowded in Taverley Dungeon, but a little far from the bank.  Catacombs of Kourend - the best place to afk, and close to a bank. Go here if you are going to melee.   Taverley dungeon is the best spot of all if you are going to go for a maximum amount of experience. There are 24 spawns, and you can place a cannon. That makes for some of the best slayer and ranged experience you can get.  The catacombs do have another major advantage when you take into account the tertiary drops that they have. There are two that really matter, besides the ensouled demon heads you get there.  Totem pieces - There are three totem pieces you need to collect before you get a complete one. The base, the middle, and the top. All three pieces are pretty rare, and you have to get them in order. Once you complete the totem, you can use it on the altar of Kourend in the middle of the dungeon. This will transport you to Skotizo, which is kind of the boss of this dungeon and counts as a black demon on a slayer task!  Ancient shards - These shards are pretty useful. You can use three of them to give your Arclight a thousand charges. A charge gives you one swing with the sword. The Arclight is one of the best weapons to use against demons. The weapon is very useful against Skotizo and the Zamorak General in God wars. It is a good idea to save them for those two and use a different weapon against Black demons.