Never enchanted in Skyrim but in Oblivion black soul gem was da best. I think creatures soul is worth more than humans, depends which creature of course. Try catch different souls and see which one is the best soul. ''Gotta catch'em all''

In both Skyrim and Oblivion Black and grand souls are equal strong, black has the benefit that you can fill them with any npc and you run into lots of bandits at level 1, while grand require mammoths or some boss monsters you find deep in an dungeon at high level.