Out of the warmongerers only Shaka really worries me. Most of the other warmongerers in my game don't actually end up doing anything tbh. Sure, if they attack you in the first few eras, they can cause you grief, but that is usually easily handled through good diplomacy. Once they start to lag behind in tech, they're never a problem again. Shaka seems to keep up well enough on tech...primarily because he has a large empire (his UB helps him sustain it) because he took out an opponent early on, kind of like a human player
It is mostly financial civs, often on another continent, that cause me the most threat to legitimately win the game. Alternatively, it is civs that tech well enough to get to the renaissance in a timely manner and then come out swinging...hard. France and Russia in particular (mainly Nappy and Cathy) are good examples of this.
If I had to pick one leader from the warmongering club, it would be Shaka hands down.
If I had to pick one leader from the tech club, it would be Mansa hands down.
These two consistently do the best at what they are designed to do. Mansa is generally a threat to win via space and Shaka is generally a threat to DoW on you and screw you over (despite the fact that he may not be a threat to win the game himself).
Most financial civs though will cause problems. Ragnar (fin/agg), I don't know why, but I've never really had a problem with him. He seems easy enough to manage via diplo and for a warmongerer, he doesn't really cause me as many problems as others seem to report.