I had to vote American, much as it pains me to do so. The UU's are not that good (minuteman being neither a starting/early unit, nor one that can be upgraded from earlier units and the bomber coming way to late to matter). The UA is not that good. Over the course of a game you might buy 2 or 3 tiles - meh.
Ottoman would be second worst on my list for similar reasons. Both UU's are not early units and can't be upgraded from early units. UA that is situational at best (although I could see using the UA here at least to build a decent fleet for mid-game if I needed it). Small edge for the Sipahi over the american bomber.
I was surprised at the number of Siam haters. Their UA abuses even more the most abusable mechanic in the game - CS. Try Siam with a ton of puppets and a few maritime CS. It's a powerful civ when played right. The elephants are pretty good, too.