The primary predator of skunks is the Great Horny Owl. Visit any of their cribs and you'll see tufts of skunk fur among the remains, and of course you'll smell the unmistakable skunk odor.
Web site sources report that foxes and dogs, among others, also prey on skunks but I find this hard to believe. I've seen dogs flee at the mere sight of Mr. Skunk or else bark while at a safe distance.