Diplo Victory is the easiest to get by far. So easy that I often turn it of just because it becomes an auto victory for me most of the time. Also, with Gods and Kings gaining city states has become easier than ever thanks to the new way that city state missions are handed out. Generally I have every city state on my continent as an ally by the start of the renaissance and every city state in the world under my control by mid industrial. I always have such a huge influx of gold that no one can budge me from that position. A thousand gold is ususally just a couple of turns worth and the AI can't stay focused enough to compete in a bidding war.
[sub]Immortal difficulty by the way[/sub]
[sub]Immortal difficulty by the way[/sub]