Nurburging Nordschleife:
6':08" with Mazda Furai (Vette's tune and a little bit spring mods)
6':09" with Pagani Zonda R
I personally use Laguna Seca as my favorite 'test' track:
59.919 for the Ferrari F10.
6'00 nordschleiffe with Tommy Kaira ZZ2
fastest non-race car in the game i think...