Germany and Persia for me.
In a recent game, German troops marched into my peaceful Russia nterritory - I demand they get out, of course, war. I kill all the invaders losing no units myself and make peace. On the first German turn after the peace treate, they move knights in my territory. I demand they get out, WAR. I kill the invaders, take two cities and make peace. The very next turn, Germans come knocking again, and it's war once more. Aargh
And of course, they always try to stiff you in tech deals and are always annnoyed when you meet them. I hate them ( the computer pixel versions, obviously). They are predictably infuriating.
The Persians on the other hand just can't be trusted. In another game, I had peace with them for a long time, paying tribute
now and then while buidling my empire, eventually allying with them 2-3 times. The wars end, I pay them for a ROP to get my troops home, they go Gracious.... and promptly sneak attack. Bah. The city they attacked later turned out to have coal in its borders. It was also my only harbor, so all my trades were broken and my reputation shot to hell. Much in the spirit of this game, I proceeded later to ambush their entire offensive army which was enroute to another civ's city and then burnt all their cities to the ground
And I so prefer to play an honorable game ...