I'd say Robin Hood, as mentioned earlier in the thread. Robin Hood has been full on re-imagined (from Foxes to Costner, etc), and rebooted...amply. Robin Hood spreads across just about every type of media through many iterations of pop culture from ballads to comic books.
The Arthurian Legend is probably up there too, but it gets diluted somewhat because there are many cases where only one or two characters or a some plot thread is stolen for use elsewhere. On the other hand, Robin Hood is a single dude...and just keeps popping up about every decade or so in SOME form or another since the 15th century.