I say the bobcat, at least to my flock, is the biggest threat. In fact, a bobcat took my Buff Orpington hen yesterday. They are the only animals large enough, stealthy enough and fast enough in our area to actually kill our chickens. Especially jumping on tall fence with a 10 pound hen in its mouth. Raccoons, skunks, rats, mice and Opossums can't get to the flock at night. Hawks are a threat as well but chickens can see it coming before it attacks, or before it even sees them! A mountain lion probably would be tranquilized before it came near our house. No bears in San Diego (thank goodness). Coyotes can't get in the yard because we are on the edge of a steep canyon. A coyote would need a running start to jump a fence. The owls here come out at night and are too small to catch chickens anyway. We dog-proofed and snake-proofed our yard years ago, so that's done. No human threat so far, except for accidental kicks. Domestic cats used to be a problem when my chickens were babies, but now the cats are afraid of them.
The one thing that wasn't put on this list that I think should have been is the crow. Crows are a MAJOR threat to chicks. When my white leghorn lily was a chick she was almost snatched by a stealthy crow. When my rooster and his sister were chicks, I took a break from watching them for one second just to grab a snack. I still had my eye on them through a window. As soon as I stepped inside the shadow of a crow loomed above the chicks. As it began to descend, the chicks began to run for cover. I burst out the door, yelled at it and scared it off. From then on though, crows were watching and waiting for me to let down my guard, kind of scary really.