So I did a bit of research and this is one of best articles I found for anyone else interested or just plain curious.
There are two answers here, of course, the short version and the long version. The short version is simple: Rolex and Patek Philippe.
http://www.watchtime.com/blog/fratello- ... -in-value/
Also this is a good read:
http://time.com/money/4058109/watches-v ... nvestment/