--I don't have problems with Kublai. He has AGG melee plus a UU HA with bonus xp from his UB, add to that he is CRE so, as he captures cities, he can use the land sooner.
--I think you will find, most on here will disagree with you on that. The NC has a 5 base attack vs. the 6 base LBs, with City Def, they don't stand a chance.
NC are like dogs and Jags, Great at harassing workers and units outside a city, but, their diminished strength makes them worse at attacking cities. More seige will be needed.
--I'll grant you, that, he isn't as unpleasant as others. As for playing as him, he isn't that good. He is PRO (we already know that's the worst trait) and he's EXP. EXP can be good mixed with ORG or FIN, but, in the long term, it's benefits to granaries and harbors are mostly for the early part of the game. Harbors, you can't even build on interior cities, and I usually don't have a problem getting health resourses.
--I would prefer Charlie to Mao myself. My cities would be started sooner and I would have better units with all of those GGs. His starting techs may not be the best, but, all it takes is a GS to pop a tech, and trade for the ones you don't have.
Again, I am not saying Charlie is Great, I think his plentiful GGs will give him better melee, mounted, seige units than say Mao.
--Fighting against him, isn't too bad, it is playing as him. His PRO/SPI traits and his UB don't do much for adding Gold to his coffers.
Some of these might not have met your criteria for "worst", if, you are referring to leaders that build huge SoDs and attack your cities, then, no leaders like Saladin wouldn't fall into that group. They would fall into the "worst to use as a player", group. So, it depends on your definition of the word "worst".