Synonyms for Astonishing (same or very similar meaning)
WordNet sense 1 (surprising greatly):
amazingWordNet sense 2 (causing surprise or wonder or amazement):
surprisingWordNet sense 3 (so surprisingly impressive as to stun or overwhelm):
astounding, staggering, stupefyingWordNet sense 4 (making a strong or vivid impression):
impressiveFrom the ODE community, based on WordNetadd/edit