The thing about Donkeys and Llamas is they are also prey animals. They will run off smaller predators like a fox, dog, or lone coyote, but they are going to run at the sight of a cougar or a pack of dogs. They aren't stupid! *L*
I agree with A1 about donkeys. They are wonderful animals, but notoriously willful and can be pretty pushy with smaller livestock (though I do know people who have successfully raised donkeys with sheep and goats and they do just fine.) Donkeys also give you benefit of their loud bray. So if you're around on the farmyard most of the time, that can be a very good alert for you even if it's not something they can handle.
Llamas tend to be more aggressive about things, and if you get several of them, they will work together and likely be more brave about rushing a larger predator or group of predators off.
Emus - Are still pretty much wild animals and highly unpredictable. I dunno about their guarding capabilities, but I wouldn't own one because the risk of getting kicked with them is just too great to be worth it to me.
Personally, I'd still stick with an LGD.