Originally Posted by larryace
But is there a definitive list that is generally agreed upon that anyone knows of?
There is no definitive list, because it's different for different people. Certain people are pre-disposed to understanding certain things, or moving certain ways. For instance, the first time I sat behind a kit, I could play basic beats, with mild independence (meaning that I could play like a basic rock beat). But even after months of working it every day, mandolin just never came easily to me.
I would say that, even with that fact, there are instruments that are generally harder to play than others, but they are not the basic ones you are mentioning. The two that come immediately to mind are harp and sitar.