They're all annoying
But these are my least faves, in no particular order:
Ramkamhaeng - expansionistic AI who is a nightmare to have as a neighbour. And he will buy ALL the City-States. The elephants are annoying as hell, too. Like medieval tanks.
Gandhi - he's always a threat, but somehow very difficult to provoke into a war. His elephants are somehow less annoying than Siam's, in part because he seems to be less adept at using them. He's also a wonder-spammer extraordinaire, which I hate, because that's
my job.
Oda Nobunaga - exactly the opposite of Gandhi and a favourite proponent of the Friendly-Backstab-Friendly-Backstab routine. So tiresome. Never seems to do any real damage but will love to get you embroiled in a pointless war which will result in the loss of countless turns of production. Montezuma is another fan of this 'strategy', but for some reason I end up with Oda as a neighbour more often, so he makes my list.
Sejong - a personal bugbear of mine as the only two times I've actually lost (as in, the only two occasions when I knew the odds weren't good but fought it out until the defeat screen) were both to him. If I work out that he's on the map somewhere, I try to eliminate him as quickly as I can, despite being a peaceful player. The fact that both of his UUs are geared at defence makes taking him down that little bit harder, too.
Ramesses - less because of the wonders, and more because he seems to change his diplomatic status towards me as often as lesser mortals change their underwear. One minute he's Friendly, the next he's Hostile and taunting me for no good reason, then he's Friendly again...! I've never really had problems actually dealing with him, but he's a pain. And another City-State-stealer.