Always Napolean; he seems friendly at first towards me, but he always does some passive aggressive crap that annoys me to no end. Like sending great prophets of his religion into my area just to annoy me, or purposely settling in lands next to me, even if I wasn't planning on directly settling on those lands.
Or he'll amass a huge empire, and get all pissy at me because I decided to expand my borders or settle a land he 'had his eyes on' and denounce me.
God forbid I try to set up any sort of trade relations or research agreements with him either. He will get all pissy and refuse to do any thing when I offer it, and than as soon as he wants something he'll ask for a ton of my resources in exchange for something I don't even need; like crabs for gold, iron, tons of money straight up and for each turn. If I don't give into these demands he'll continue to denounce me and eventually go to war with me.
When ever he gets land happy I always make a point of taking his cities and just giving them away to other nations who are in need.