This is an easy one. Without this guy, the guitar might NEVER have become a popular instrument. Everyone that enjoys any kind of guitar music can thank Andres Segovia for bringing the instrument into pop culture. If I remember my music history correctly, the guitar was the favorite instrument of the aristocratic society, up until the pianoforte was developed around 1700. Then, it became the instrument of choice. It was also about this time that Gypsy clans took up the guitar, which led to its status as a lower class instrument. Few pieces of good music exist for the guitar from around the early 1700's until Segovia took up the instrument.
The Maestro