I always run the worst tyres on the front.
If you have your best tyres on the front, then you get no feedback when the rear tyres are about to let go. If the worse tyres are on the front you know that when they start to slip the rears still have a bit of head room left.
Its not just about tread either, a friend of mine baught a corsa, which had two almost new ditchfinders on the rear, and two nearly bald pirellis on the front. First time he took it out in the rain the back end stepped out without warning on a damp roundabout and after doing a 180 spin an astra crashed into him.
He repaired the damage and swapped the wheels round, and it was only then he realised how **** the ditchfinders actually were, because they were now attached to the steering wheel and he could feel just how little grip they had in the wet.
So grippiest tyres on the rear!