I agree that Earth Rave is better than Cosmo Memory, but I will stick with Stardust Ray because it has six hits and is non-elemental. I'm not too keen on Earth Rave because the animation isn't impressive, it hits only five times, and each hit is a different element. Accidentaly curing an enemy doesn't rate too highly on my agenda.
Yuffie's best Limit Break is Doom of the Living, and can be comparable in damage to Cloud's Omnislash if she is suitably equipped.