Definitely Toku, from experience. Unless you have a common religion, you won't have open borders, and you are not likely to have a common religion since the sucker will probably manage to take one and never ever ever ever ever build a road, not to mention a missionary to spread it to you. You can either wait for 1000 AD for his religion to spread or found one yourself and spoil a little bit more your relationship with him. You can't easily rush him with his traits of warfaring doom, and he will flood you with spies. Even if you do manage to get him pleased by giving him tons of techs, he won't trade anything to you ever, being usually awfully backwards and not wanting to trade any crappy tech he discovers since he thinks he got some monopoly. And finally, he will dow on you no matter what and slow you a little more. You start with him on a landmass, you're almost guaranteed to be the most backwards continent on the map until Optics. Then, he will see everyone as his worst enemy, and the other way around, so you'll have to take him down or deal with an angry neighbour all the game long because you've "traded with his worst enemy", no matter who you trade with. The man is a nightmare.
Sitting Bull would probably be a worthy second, but I never started isolated with him.