The hardest I've ever seen are GPM 55deg slicks. However, they're not popular so I really don't know where to find them other than to order them straight from GPM (which I did, a long time ago).
Mini-Z Autoscale tires are a little stickier than GPM 55's, but their tire profile is flat so they're actually quite a bit grippier than GPM 55's. But, they're easy to find, just ask someone with an Autoscale collection. You might even get them for free.
