Z06 or C6 (which may change once the C6 Z06 comes out). True, while the STi/EVO are good cars, they really aren't on the same level as the Vette. 0-60 and 1/4 mile is comparable (due to their less HP but 4WD), but what about long straights on road courses? Not to mention the better aerodynamics of the Z06, and much better handling.
Also, the Vette is a MUCH better platform to build upon. The EVO is already heavily tuned, and it's somewhat the same for the STi. I woudln't be surprised if heads + cam C6s put out close to 550 HP at the engine. Also keep in mind that the two 4-dr compacts were a basis for their respective companies to build rally cars upon.
If you want straight line only, get a foxbody Mustang and put a few thousand in. You'll be running low 10s or even 9s in the quarter.
I love imports; I was actually a die-hard Supra fan. While I still love the Supra, times change... and the Vette really is the best bang for the buck right now.