I voted for Bach's, because I generally play democracy. As other people have mentioned, the relative value, of course, depends on what kind of game you're playing. Early landing players like to have one large SSC supplemented by small helpers, so they prefer Newton's. I like getting Leo's for the ship and engineer upgrades, but I survive without it just fine. As many players have mentioned, the money saved by Adam Smith in a turn tends to pale in comparison to the payoff from a single caravan/freight. With MGE, Eiffel tower is completely useless, the AI always hates you. I've definitely used the Statue to turn fundamentalist early, and Darwin's is useful for when I have extra gold. In any game with heavy caravan/freight traffic, I tend to BUY wonders of the world, as I'd rather be delivering all of my commodity caravans. As long as the payoff is more than 200 gold, it makes more sense (4 gold/shield to rush a wonder), and that's not even counting the beaker bonus. If one equates a beaker to a coin, caravans only need pay >100 coins for it to make more sense to buy the wonder than build it with caravans. This, of course, is assuming that the city in question still has commodities to supply. If it doesn't I'll use food caravans for wonders. All that being said, I've been known to go for as many wonders as I can just for fun, as all of them are useful (except for that Eiffel tower).