it really should be broken down between electronically controlled and good old mechanical systems...it makes a huge difference...
electric would be the Prodrive P1 prototype car with fully active diffs. Production would be the late porsche system...yes, better than the attessa system.

straight mechanical I would say subaru jdm 6-spd *(because of tranny strenght and R180 rear). It has a nice clutch rear, torsen front and tight (65%rear 35% front) lsd center diff. That and its symmetrical keeping torque bias in the diff completely free from half shaft weights.

But I must still note audi for making awd mainstream...not the original but the one's who made the big impact.