I find the Gruppe M V2 exhaust to be the highest pitched. At high rpm's it sounds almost like a superbike! And since it requires removal of the cats, it's fairly loud too.
I've heard Comptech, RM, RSR, and Tiatec JGTC. To my ears, Gruppe M is higher pitched than all of those. The next closest would be the Tiatec, which is very nice sounding too.