¿Dónde están las mejores olas? (Assuming you're talking about ocean/sea waves. The other word for wave, "onda," might be acceptable in place of "olas" depending on which dialect you're speaking, but I believe "onda" usually means something more like "ripples" in the context of water).

If you want to know where are the best waves you can say: ¿Dónde hay buenas olas? Another way to say the same is: ¿Dónde está rompiendo mejor? The second is more cool than the first if you are in the correct context Boas ondas!