Washington, no doubt. On Immortal he poops out cities like there's no tomorrow.
I played France on a small continent with just Washington, Shaka Zulu and Gustav Adolphus and I actually had a lot of room to myself and good relations with all my neighbors. Then Washington starts dropping cities out of nowhere. And Shaka starts taking them, after having finished off Gustav. And then he declares war on me. Needless to say, I was boned pretty hard.