It's hard to say, actually. Because of the recharge turn, your effective power is 75 per turn (on average) per turn. But there are not many other good Rock moves. Stone Edge is very common and one of the best, with 100 power (but a bit less accurate). Head smash is the same power (150) but you get recoil.
However, Rock Slide is the same "effective" power at 75 and it means you can switch out on the next turn if you like, so overall you should go for Rock Slide or Stone Edge.
Note, for other similar moves like Hyper Beam or Giga Impact, there are much better moves you can use, like Return. So never use Hyper Beam*!
*Except for Porygon-Z and Choice Specs/Scarf Keldeo.
