I'd like to start by pointing out that you don't need a mod to create mob-spawners in minecraft. Any spawner, if right-clicked with a spawn egg will become a spawner for that mob. I don't know if you've tried making an enderman grinder in the overworld yet but you're going to need a whole lot of teleport-proofing with that one and I don't think I'd attempt it anywhere other than very high in the air or deep under an ocean. That said, some of the best hostile mobs to make a grinder for IMO would be creepers, ghasts, wither skeletons and possibly witches. If your mod lets you make a spwner for something like iron golems then that would be a good one too. Ultimately it's a matter of what you want to produce but since creepers can be used to produce mob heads and music discs as well as exp and gunpowder I'd call them the best.