After Pokémon Sun and Moon I was planning to play Black and White. This got me wondering. Which HM out of Surf and Waterfall is more powerful in Black and White and Black and White 2?
Another thing to consider when using Surf and Waterfall is that Surf is a "Special" move and Waterfall is a 'Physical' move.
What this means is that when you attack with Surf, the damage it does is based off of your Pokemon's Special Attack stat and the defending Pokemon's Special Defence stat. With Waterfall, it's your Pokemon's (regular) Attack and the defending Pokemon's (regular) Defence.
So, for a pokemon like Gyarados, you would be better off using Waterfall as he can make use of his higher Attack stat to deal more damage than he would with Surf.
This makes Surf stronger in terms of raw damage dealt to the enemy, however Waterfall may cause a pokemon to flinch which makes it stronger in certain situations, especially if used by a pokemon with a high speed stat since it may allow for an additional "free" attack.