My wife and my 11 year old son and I usually play 3 humans v 3 AI v 1 AI on Extreme.
The AI is usually really good with siege.
I am not good with siege.
I struggle vs AI civs when they have a good siege bonus.
They ram down my trebs and onager my archers like nobodies business.
They even send out villagers to repair their siege.
No human can do that.
Houfnice’s are nasty too when the AI controls them.
I usually end up playing cavalry civs to counter their siege, so camel civs are annoying to me also.
The “scariest” AI civs probably depends on your playstyle and your own choice of civs.