Jim Davidson wrote:My first instinct is to say the Chromatic Fantasy.
Search for "Jorge Caballero plays Bach Chromatic Fantasy and Fugue (1/3)" on YouTube.
That said, it's difficult mainly for it's speed, and it's mostly one voice in a straight line. I think the hardest kind of music by Bach to play convincingly on a guitar would be a fugue. Couldn't tell you which one is the most difficult to play.
The fugue in the Chromatic Fantasy and Fugue is not "mostly one voice in a straight line"! It's densely contrapuntal and is viciously hard, if not nearly impossible to play at a keyboard speed with proper legato. It, most of the keyboard Partitas, and the Goldberg Variations are probably the hardest overall.