Easy. The 1275 Cooper S from the mid-late 60's. I had a fuel-injected Cooper 35 in the late 90's that had lots of engine and suspension work done to it (about £5k on top of the price of the car), and that was pretty special too. the R53 and R56 are both better cars, no doubt about it, but neither are really anything more than very pale imitations of the real thing when it comes to characteristics like handling and nippyness.