What's the most accurate sundial

A generic antique reproduction sundial bought without consideration for it's exact positioning, like those often sold in garden centers, is worthless as a time keeper. For a sundial made for its exact lat & Long the answer is about two minutes. If you want one made see: http://www.merlinsundials.co.uk/ Or if you want to make your own see: http://www.Amazon.com/Clock-Accurate-Customized-Location-Northern-Hemisphere/dp/0879612460. I made my own and it's easier than it sounds. However a larger one used with correction charts could be accurate to the second. See: http://www.sunlit-design.com/infosearch/sundialaccuracy.php?indexref=1 In fact up until the turn of the 19th cent. the French railways still used what they called heliochronometers rather than clocks.