amazing, astonishing, astounding, bowling over, dumbfounding(also dumfounding), flabbergasting, flooring, rocking, shocking, startling, stunning, stupefying, surprising(also surprizing), thunderstriking

befuddling, bewildering, blindsiding, blowing away, confounding, confusing, dazing, discomfiting, disconcerting, dismaying, jarring, muddling, nonplussing(also nonplusing), perplexing, shaking up

knocking for a loop, taking by surprise