any technique that's not yours is the hardest to play...because if you stumble upon it naturally without striving for it, how can it even be learned? it is just picked up in that way. If you want to go for who has the most elaborate and impossible to emulate technique, it would depend on what style you're talking about. Jimi and blues/r&b/rock/etc? gimme a break, no-one has a rat's ass chance in hell. Jazz? McLaughlin will dispatch your sorry ass time and time again for as long as you live. The most impossible is definitely John McLaughlin, the all-time grandmaster of grandmasters on the guitar. A lot of guys play sweeping patters with sudden pauses and direction changes, but at the heart of it they are playign sweeping patterns and areggios and the whole lot - McLaughlin is the most jagged stop and start plus CONSTANT IMPROVISATION ON AN ALTOGETHER UNATTAINABLE SCALE - it's not a stone's throw away, it is not even worth your time trying to learn how he does it...even with his new DVD coming out in 2003, where he will be breaking down all his technique and styles he derived over his lifetime, elaborating and shedding his every trick, everyone will be in the dark for he still has that improvisational edge that basically only Jimi can match as far as modern guitarists. It is finished...that's the sign of the greatest kind of artist - the one who spills EVERYTHING for you, each and every little bit, but you still have no chance at getting there...