i think the hardest would be fusion stuff..technically and mentally challenging. you must improvise technical riffs on the spot given only a single motif.
I agree - fusion should be the most technically challenging - one of my top choices is anything from Tribal Tech - check out Willis playing "Got Tuh B" or "Slick" and listen to how wickedly tight, smooth and fast he plays...amazing stuff.
i think the hardest would be fusion stuff..technically and mentally challenging. you must improvise technical riffs on the spot given only a single motif.
I tot the bassline from Anna Molly (by Incubus) was quite tricky. Technically speaking, its not difficult. But imagine playing both a bass solo and a guitar solo together, and when the guitar solo screams for more attention its quite distracting to play the bass solo.