Hands down US
Shred as I see it (technical mastery rather than a particular style) has more complex origins, you can't deny that if you plugged Djanjo Reinhardt (France) into a few modern pedals the man could shred - all the more amazing as he only had 2 working fingers. Throughout the decades you had others who took the guitar to new directions, no one mentions Chuck Berry or Les Paul from the 50s. Then of course Hendrix, a lot of people tut tut this, but he was different from his contemporaries. The first shredder of the neo-classical style was Uli Jon Roth (Germany) of the Scorpions, followed by Ritchie Blackmore (England), they were again head and shoulders above their contemporaries.
Then in 1980 Steve Vai started playing for Zappa, FZ called it stunt guitar, but it was shred. Consistently the US has produced more exciting shredders than anyone else. I feel by far the biggest contributor is the US's position as the largest cultural melting pot in the world, think of every genre that is popular and it was formulated in the US, sure the origins are African & European, but the place that makes it happen is the US. I liked some great European and Asian Guitar players, but the ones that really inspired me to play were always American.
Jason Becker, SV, JS
, PG, MAB, Petrucci, Marty Friedman, Tony Mac Alpine, Vinny Moore etc.,
Britian has produced some great players that I love - Guthrie, Ritchie Blackmore and one of my all time favourites...Brian May, this little island has produced none, so there were local players I liked such as Gary Moore and Rory Gallagher, but the altar I sacrifice my virgins at is to the pantheon of American guitar gods....