Single Ladies was huge in Europe at least, but Gangnam Style was definitely even bigger (and this is coming from a part-black person). Single Ladies was the closest to a "phenomenon" that we had in 2009 though.
Single Ladies is bigger and will remain bigger. Gangnam Style is just like LMFAO's "Shuffle" that will be forgotten in a few years. Besides, Bey created the Gangnam Style move.