What do you think is the worst insult possible?

That depends on geographical location I would assume. Like calling someone a uneducated fuck in North America is a pretty nasty insult in my opinion. Basic education is readily available and in most cases free, also resources that allow someone to expand their knowledge of many things is readily available and fairly cheap. Some resources are even free. So to stay ignorant and uneducated in this part of the world is pretty difficult and takes a lot of talent. Granted, you are not likely to become all knowing about all things. But you can still obtain basic knowledge and understanding in most subjects, fairly easily.

Depends on the target's particular weakness. I.e if they're concerned about their weight, height, lack of looks, lack of money, birthmark etc. But sometimes the worst insult is just ignoring them so they can't vent their argument.

Calling someone Trumpesque

To me the worst insult would be for someone to say that I'm just a bunch of chemicals and that I have no purpose to exist. But even worse would be for someone to say I came from an ape- I'd much rather be a splat of chemicals.

'you aint ma friend no more'

TrumpLover.

Average.

liar