I think my first experience with in-game racism started with Alexender/Greece. It was my first experience with an aggressive AI and although he didn't directly attack me (until later) he decided upon himself that he'd like to conquer everyone around him and it usually works. Then to add salt to the wound if you want a city state ally with Alexander in the game, you'll empty your pockets dry. Just to find that he's now their ally again.
The second contender would definitely go to Shaka, Attila or Genghis. At least Alexander has a plan when he wants your land, these guys will just DoW and be a general nuisance even if their UU spam can't take your land. Oh what's that? You're the most behind civ in the game now because of all the unsuccessful wars you waged? Now they're brown nosing you, asking you for help whenever they see fit with a big old "Friendly" tag. Only to shove a knife right in your back when they've decided that you're weak, or they've just gotten bored.
Then last, but certainly not least is Sweden. Sweden is subtle about it and that's what makes it so much worse. They'll get nice and friendly with you with their DoF's to benefit from their UA but, the moment that they think you've done the slightest wrong by them or they think you're weak, they will swarm you with the biggest army you have ever seen. Sounds a bit like Russia and England now that I mentioned it, but at least they have the decency not to leach off you from their UA.