If not the judge, then...
The first time I picked up 10 little Indians to read, I just KNEW the judge was the killer, I don't remember why, just from the start there was something about him, which of course you get thrown off when he dies, but then at the end when you find out it actually was him, it's just that feeling of "I knew it all along!", somehow it just seemed too obvious the first time in. He almost seems too likely to be the guilty party, so, book or movie, if the judge hadn't turned out to actually be Mr. Owen, who do you think it would've been?
I can't say for the book, but in the movie, Emily Brent would get my vote because she's a perfectly dissociated, heartless, sociopathic bitch.