Surprised everyone is saying China. They tend to be on the lower rung of moderate in most of my games. As for the worst ones, in a word: England. England, always England. It looks like she keeps going for culture victories or something, which I'm not sure is something that makes sense for them. At least I'm guessing they do, they tend not to survive past the Renaissance so I don't really know what their strategy is. All I know is they build two cities and just kinda sit there until someone kills the crap out of them.
To a lesser extent, America tends to suck too. but nobody out-sucks Elizabeth.
To a lesser extent, America tends to suck too. but nobody out-sucks Elizabeth.