re: Guitar guys??? BEST training exercise?

Every scale you can get your hands on. Run them in each mode, in each key. Single and multiple octaves. Arpeggiated. Run them by interval, i.e. root second, root third, root fourth, etc. Run them in broken seconds, third, fourths, etc. Run them in cycles of fourths and fifths. Work on extending the chords scalarly, i.e. Major, major seven, major eleven, minors and diminished. Practice the arpeggios and their inversions in a scalar fashion. Work on your ear. Recognizing scanned, modes, interval and chord qualities. Work on your reading.