Stunningly beautiful is the highest compliment I can think of. A close second is breathtakingly beautiful and then after that is extremely beautiful, very beautiful, beautiful, attractive, pretty, cute, nice-looking they're all the same level at that point. I could only think of using that on someone who appeared to me as 100/100 or something attractiveness. The type of beauty that literally just stuns me in place. Like I can hardly believe that it is real. I don't think I would use it easily or something
I would say absolute stunner if she was so attractive that I know all other guys will agree. If just I thought she is attractive I would say "I think she's hot" but if I know everyone else will agree I would say, "she is an absolute stunner."