If you take out the City-States, it renders the Unique Abilities of some Civs (i.e. Siam) completely useless and severely hampers others (i.e. India, who greatly benefits from Maritime CSs).I know what most people are going to say, but I reckon it's the Mongol Terror. There's no reason at all to attack city-states, so it's redundant really
I'd personally say Seven Cities of Gold is the worst simply because it's so rare to get land next to a Natural Wonder. The early gold is nice, but again, you're not always guaranteed to find natural wonders quickly, either. At least with other early gold producers (i.e. triple money for destroying barbarian camps), you know those are going to pop up frequently.