We decided that D2's official first word will be "Uh-oh." It's a little hard to determine among the many purposeful babblings which sounds are truly words, but we're all pretty sure on that one, and he says it quite often. Also, it suits his personality just fine.
The first time he said it distinctly, we were all sitting at the table coloring. D1 dropped one of her crayons on the floor, and said, "Uh-oh." D2 promptly tossed one of his crayons on the floor, too, and said, "Uh-uh."
Almost equal in use, however, is the phrase "up-down" (or "Uh-dah" but the inflection mimics mine). Since he uses the two together, I think they may only be one word in his mind, perhaps meaning vertical movement. He uses it for doors on the puzzle board, for clothes he's tossing out of a basket, and even as he stands and squats in turn, for all the world like a miniature aerobics instructor.