03/04 Cobra is hard to beat.
This might not be affordable now, but given German cars' and their resale value, in a few years this is the new numero uno, B8 S4 stock = 333hp crank, + chip = 431awhp. There is still room for improvement with pulley and exhaust.
EDIT: Wait, cobra's are like $20K+, there are better bang for the buck cars...