I'm not convinced that dynamic clothing needs to be parented at all, provided you start your simulation from the zeroed pose. I never do it - but if I did, I'd say the neck would be the logical place to parent a cape. However, don't be misled into thinking that parenting will stop your cape falling down when you simulate, because it won't. For that, you need to constrain the part which attaches, as xantor said. Exactly what you need to do will depend on the design of the cape, i.e. whether it has fastenings or not.