I can't say why there are no responses other than this question is asked regularly, and much information is available through searches. There is no "right" answer for the question as there are many variables. High performance, all-season, winter... Many (including myself) use the Pirelli Scorpion Verde all-season as they are factory used and wonderful. Many use Michelin and other's select other brands. Continentals are also very popular here in the U.S. I would never purchase tires for a luxury high-performance vehicle based on price (read: cheap).
If you're running factory 255/55R18's then there are only a few tires in each category and all are great. When in doubt, check the approved tires by Porsche and you can't go wrong.