Ferrari GTO obviously
http://www.bloomberg.com/news/2012-06-0 ... llion.html
I've never thought of cars as an investment, but the S1 Exige is a lucky buy.
Assuming you wish to drive the car, I think just looking for something that will not depriciate should be the main objective, and also consider something that will always be in demand. So I'd rule out anything ugly or in the wrong colour or spec.
From my limited knowledge of the Porsche market, the price range is vast even for the same model. Condition becomes all important. So the old Carrera model can range from £10k to £35k, purely dependent upon condition, refurbs etc. Certain models are less desireable and will always remain so IMHO, so if it's cheap today, it'll probably be cheap in the future.
At the end of the day I'd buy it because I wanted it, at least then your not too disappointed if it costs a bit to run.