I have never had the honor but I have heard that the .55 Boys anti-tank gun was the recoil king back in WWII and may still hold that title.
http://www.youtube.com/watch?v=PlMTyCOWg-k&feature=related
http://www.youtube.com/watch?v=ov4lOcD7O5c&feature=related
http://www.youtube.com/watch?v=PlMTyCOWg-k&feature=related
http://www.youtube.com/watch?v=ov4lOcD7O5c&feature=related