For me right now it is warren.

I'd say Warren. Historic is hard, but there aren't many missions there and you're pretty OP by the time you get there

I'd still go with Historic. You are OP by the time you get there, but all of the enemies take several shots to take down (unless your precision them in the face) and can do a fair amount of damage if they go unchecked. Also the blimp mission pissed me off.

Landmines on the roof combined with Uber-Soilders combine to make the Historical Distric hard. The little trash monster scorpion things on the Warren are annoying though.