To say that Django Reinhardt is underrated is really not true. It's only in today's fast food, get it now culture, his talents have been ignored by the mainstream. In his day Django Reinhardt was considered by most to be the best guitarist in the world.

A short list of other worthy artists (In no particular order) who may have fallen off your radar

Son House (Delta Blues)

Andre Segovia (Classical)

Fernando Sor (Classical Composer, no recordings however his talent was legendary)

Scotty Moore (Elvis' guitarist...Lots of tasty nuggets in there)

Shawn Lane