Which tyre is best?

[racer_d]

Henrik - "best" for what?

If normal, spirited street driving, as Porsche intended, in most elements, then follow the porsche suggestions. Since you are in the UK, I am not sure what other brands you can buy that have been mentioned here already (ie Khumo, or Toyo)

I am not suprised that your car currently feels slippery given the fact you have mixed brands of tires. I am sure tread patterns and likely tire constructions are different between the two. You don't mention how many miles you have on the tires, but the rears typically don't last more than 15,000 miles. Fronts can take as much as twice as long to wear out. Obviously, tread depths will impact their wet weather performance.