WordNet (r) 3.0 (2006):

breathtaking
    adj 1: tending to cause suspension of regular breathing; "a
           breathless flight"; "breathtaking adventure" [syn:
           breathless, breathtaking]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "breathtaking":
   agitating, amazing, astonishing, astounding, charged,
   cliff-hanging, confounding, disquieting, distracting, disturbing,
   electric, exciting, exhilarating, eye-opening, galvanic, heady,
   heart-expanding, heart-stirring, heart-swelling, heart-thrilling,
   impressive, inflammatory, intoxicating, jarring, jolting,
   maddening, mind-blowing, mind-boggling, moving, overcoming,
   overmastering, overpowering, overwhelming, perturbing, piquant,
   provocative, provoking, ravishing, soul-stirring, spectacular,
   spirit-stirring, staggering, startling, stimulating, stimulative,
   stirring, striking, stunning, surprising, suspenseful, suspensive,
   tantalizing, telling, thrilling, thrilly, troubling, unsettling,
   upsetting