Wow, that thread didn't get very far!
My favourite designer is Marcello Gandini because he pushed the envelope that much further than anybody else and it's much easier to fail than succeed when you stretch yourself this far out.
A close second for me is Giovanni Michelotti, albeit for very different reasons. He accomplished so much in his career, with such grace, creativity, elegance, and building up a distinct image for many manufacturers who benefitted greatly as a result. Also, I cannot think of any other designer who managed to work so well on cars of very small dimensions. It is one thing achieving good balance with long, low-slung chassis, but it's so much more difficult when you are limited in your creativity by a very functional package, and Michelotti excelled in all areas with equal genius.
There are many others I would like to mention of course, from different eras and horizons, but with just these two you already have a significant chunk of automotive design history covered!