I absolutely hate Korea and Kongo. These two civs are usually the absolute worst runaway civs if left on their own, and they are the reason I would really like an in-game option of banning certain civs from my games. I don't mind if they are my immediate neighbours (I can conquer them before they get a massive lead) but if I discover them in late Medieval era or onwards, that's going to be a very rough game as they just snowball through the tech tree like nobody's business. Somewhat true for facing Russia in a religious game as well, but in that situation I can always transition to a different victory condition, and religion by itself doesn't snowball out of control like science does. I like Sumeria - always an easy friendship early, and discovering him as my neighbour means I'm safe early 90% of the time.