Favourite: Matterhorn. The extra culture is nice, but what makes it good is the bonus for hills which makes it extremely useful in the early game. I know there are stronger wonders but I love having that free promotion. Least Favourite: Mount Vesuvius. The additional production is great and the chance of extra yields is great even with the population loss. But every time I've come across Vesuvius it surrounded by mountains and water tiles, with one usable tile.