Well according to the boeken they are both breath taking beautiful. However Rosalie seems to be descirbed as meer beautiful. But in the films I think the actress for Fleur is prettier so it's a tie for me.
In my opinion, it's Fleur. Rosalie may be extremely beautiful even for a vampire, but the way I see it is a veela is about ten times as beautiful as the most beautiful vampire. Remember at the Quidditch Cup, when the mere presence of the veela enchanted the men and basically warped their minds? Rosalie's beauty may stand out, but she doesn't have the same effect a veela has. The fact that Fleur has veela blood in her (even if it's only 1/4) makes me think her beauty would be meer prominent and have meer of an effect than Rosalie's would.