Persians. Trouble with your neighbors is to be expected, but the Persians will declare war on you from halfway around the world, then send a stack of half a dozen immortals, slogging through jungles and trekking over mountains, through four other civilizations, to arrive on your doorstep 40 turns later. The threat of immortals is just enough to divert half a dozen units from more important tasks for that 40-turn period. What is the point of sending your own units on a similar journey, to force Xerxes to offer 40 gold for a peace treaty? His cities are too far away to keep, if you capture them, and there are too many civilizations in between. He should pick on his neighbors. Also, the Germans and Aztecs suck.