A few days ago I have installed a set of new Michelin PIlot Super Sport. I've decided for 235/45R18 front and 265/40R18 rear on stock Z4M wheels, and I believe it's a really good size for street use. Slightly meatier than stock - which provides better protection against potholes, and just a tad wider.
Yesterday was a rainy day, and I discovered that on 1st gear, full throttle and wet road, the DSC light says nothing. With my previous stock Conti M3 255/40R18 it would flicker even on a dry road.
The PSS is unbelivable.