Depends on the animals you're wanting to guard really. I wouldn't keep a donkey with sheep or goats because they sometimes play rough (pick them up by their necks and shake them,) but with cattle you can't get any better than a donkey. Sheep and goats I prefer a llama over even dogs (especially those nasty great pyrenese, worst lgd's on the planet, in my honest opinion.) I know nothing about emus (yet lol,) so I won't even comment on those.
Well if you're looking at a llama you can not get an unfixed male as a guard llama. They may try and breed sheep or cattle without access to females.... Llamas also get very attached. If you move the llama away from the herd or if the herd is butchered or sold, the llama will be deeply perturbed by this. Not all llamas are guard llamas. I have two female bay llamas and they don't take much of an interest in my chickens, so I don't know. Our neighbors use three burros to watch their sheep. Llamas are great pets! They are fun to own, and sometimes people stop in the road to stare at them... And emus can jump any fence that is less than 6 feet. I used to want one until I found that out... Don't want an emu running down the road.
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.