OK, so there wasn't a contest. But an anonymous commenter said, "Looks like cosmos," and from the images I found by way of Google, I do believe that's what my mystery plant is!
The only thing that does not ring true is this, since mine are eight feet tall: "Cosmos give you a big bang for your buck. Growing carefree from seed, these annuals (meaning they grow just one year) fill a flowerbed with color and are great cut flowers. Cosmos grow anywhere from 1 foot to 3 feet high, depending on the variety."