This was a hard decision for me. I've had problems with quite a few of these unfortunately. But in the end I picked coyote. Although I will say that man and his dogs are very much a problem, that's not what I would consider natural predation, that's more of terrorizing to me. I've had dogs get my chickens and I've had people steal my chickens, but that's not what I typically have to work to protect my flock from. Our coyotes are really getting out of control, as are the coons, opossums, skunks, etc. We've trapped many a coon and opossum, but the coyotes are a different story. They are very wily and quite honestly it's hard to keep one in your sights for more than a few seconds. I was out one evening with the flock at my grandparents, right next to their coop, just standing and watching all the silly antics

It was quite a large group, some fifty birds, EE, BA, and a few bantams. A couple of the large roosters that were furthest from the coop started standing tall and looking excitedly in a certain direction. I couldn't see anything and to be honest I figured they were just seeing ghosts like chickens so often do (it cracks me up every time. They see an acorn fall or something silly like that and think the world is ending). But then all of a sudden one of the roosters shot into the air! Right along snapping under his tucked up feet was a big and ugly coyote! Now that particular EE rooster was very hefty, and did very well jumping into the air like he did. Of course then it was pandemonium. All the chickens ran and flew for the chicken house while I woke up from my startled state and started yelling and picking up things to throw at the offender. Didn't have any injuries that time but it was just so scary, especially since I was a mere fifteen or so feet from the action. I mean I could see the coyotes slobber. Ick. I must have just been so entranced and so still, he didn't notice me while he was stalking. That or their getting very bold, which I don't like at all.
Years before this I had a small mixed flock of about 8 or 10 birds. We live on a hill and they were out on the pasture. A small pack of coyotes got all but two, one was a cochin rooster who never left the house and the other was a brown leghorn rooster, who incidentally is the best running chicken I've ever owned.
I've had the odd chick taken by a hawk, and once even had one come down almost within grabbing distance, just floating over a couple chicks I'd bought that were trying to hide under me. Crazy, but mostly hawks don't bother us much, especially since now we keep all our chickens at my grandparents, who have lots of brush and trees for cover.
Coons and opossum are a terrible nuissance at night if my grandpa doesn't get out to shut the coop before dark. Snakes are usually only a problem for eggs or hatching eggs, but once a tiny little chicken snake suprised me. I had a fairly small broody hen, VERY protective, about 3 or 4 pounds who was separated from the flock. She was hatching ten eggs, and they were chirping and whatnot, one had hatched, and the rest were trying. I came back to check on them about half an hour later, and a snake had not only swallowed the one hatched chick, but it was trying to swallow the mother. All it had was her head and truth be told, I didn't know what it planned to do if it accomplished swallowing her, it wouldn't have been able to move. So I lost that hen and one chick and found another hen to adopt the eggs and soon after chicks. Tragic but very odd, and it only happened once.
We used to have packrats get chicken feed but that's all they did. Skunks have come in and gotten some eggs but couldn't reach the birds on the roost.
Owls are definitely a problem here though. There are so many, it's quite eerie at night to hear them all hooting. As long as none of the chickens stay out it's fine. But I have had the occassional large rooster or banty pair decide to stay out of my reach in a tree for the night. And it's almost a guarantee that they won't make it through the night without an owl attack. Very menacing. Anyway those are our predator problems we try to put up a defense for. I read in an earlier post about platforms? That idea is very cool and would like to try it sometime...