Atra du evarinya ono varda.
It's either Surf (for special-based pokemon) or Waterfall (for physical-based pokemon) if you're talking about in-battle usage. Both powerfully 100% accurate moves. (Unless it's in a double/triple battle; Waterfall takes first place.)
For out-of-battle usage, I think Surf is very useful. It enables you to reach areas only accessible by the waterways (for example, an island in the middle of a body of water). Who knows? You might find a rare TM there, or an item you really want.