Jazz
This is the musical genre which undoubtedly provides the greatest freedom for self expression for any musician.
Improvisation utilising the theory elements (essential for any serious instrumentalist to learn), mean that this is a highly useful area to explore, even if it is not your main playing style.
The guitar as a Jazz instrument was pioneered most notably by Django Reinhardt and later by Charlie Christian. The style has evolved and crossed over into rock and pop through modern players like Larry Carlton and Pat Metheny.
Implementing harmonic substitutions, scales and scale modes over complex chords and chord progressions provides a challenge in terms of your knowledge of the instrument, compositional sense and technical prowess. Many of the scales you will learn are heard in music by artists like Steve Vai and Joe Satriani, together with many contemporary metal acts.