I assume that your main priority is dry traction.
I personally have owned the Kumho MX and 712, have the stock Bridgestone RE-070s on my STi, and I've had a number of different R-compound tires.
Take the 712 off your list. Not even in the same league as the others.
MX is a good tire and an excellent value, but it falls short of the RE-070 and R-compound tires by a good margin.
I've heard that the Falkens and the Hankooks are both excellent tires for dry traction that are in the same league as the RE-070s.
I'm leaning toward the Hankooks as my next summer tire set simply because it appears they will handle the occasional downpour better than the Falkens.
So, for dry traction,
Falken ~= Hankook > MX >> 712.