Aryton Senna: God rest his soul, but I have to believe that he would cringe at what has happened to circuits since his untimely death at San Marino.
Great circuits such as Hockenheim, Nurburgring, and Spa have all undergone changes to slow down cars and, in my opinion, make them less than great. Look at any circuit and imagine a mechanical failure such as Senna's, and you would warrant putting in a chicane.
Remember, racing is an inherently dangerous sport, but great racers deserve great circuits. I have a top five of all-time non-altered circuits and would like to hear yours. In no particular order...