Over a twelve month period in the UK, judged by trade value as a percentage of list price, a Ferrari F430 Spider maintains 97% of its value -- better than any other droptop. Next in line comes the VW Eos 2.0 TDI Sport, at 94%. They are the only two examples that manage to lose less than 10% of their value: the Porsche Boxster S and Mercedes-Benz SLK55 are both tied for third at 87%.
EurotaxGlass also reveals that while there are peaks and troughs in convertible values based on the time of year, hardtop convertibles are not so affected by them, which makes sense. But they also say dealers have reported buyers not wanting folding hardtops because then other people wouldn't know they were driving a convertible. According to EurotaxGlass, "It seems discreet coupe-convertible styling does not flatter the vanity of some owners." Click the link to get the entire list of residual values.