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

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

knocking for a loop, taking aback, taking by surprise