For what it's worth--leaders;
Monty; Always annoyed, therefore dangerous. Generally, the way I deal with him is to rush him if he's too close, or bribe him into war or other AIs into war w/him. I did have him peace vassal to me on one occasion, and he proved ineffectual. Won the game, but Monty was very little help.
Alex & Nappy; Situationally dangerous. Given a good starting position, and successful early wars, either one can be potent later on. Usually, this isn't the case. They are too aggressive early on, and pay for it.
Ragnar; Seems like if he senses weakness, no matter if you are on the other side of the continent, he will declare and send an army your way. If you manage to stay on good terms with him, not an easy thing to do, he might not DoW. I once played a game in which Ragnar was north of me on a large continent, I conquered three other AI empires, and did not get attacked. The only thing I could figure out is that, by coincidence, Ragnar was also at war with the AIs that I was conquering and by the time I was done, was too powerful for him to attack.
I have seen Saladin, Mehmed, and Mansa REX very successfully, but have also had each of these peace vassal to me out of the blue, usually when they were under attack.
Shaka; as pointed out, he seems to have the ability to tech well, expand well, and is super aggressive. Like dealing w/Monty, if he's nearby, rush him if you can. That said, I played a game in which Shaka peace vassaled to me . . . and he was the most ferocious and effective vassal I've EVER had. As a fighting vassal, he MORE than held up his end, fighting on four fronts simultaneously . . . and he won. I was prepared to send armies to assist his defense--had them held in reserve, but never needed to commit them.
These seem to be the problem children.