Mine is Scald, because it's the only Water-type move which causes a burn. It's a great combination of uniqueness and realism, but also, it is very commonly seen in competitive battling. Marlon also utilizes this move in his Gym at Humilau City.
I think Scald is cool too! I've been waiting (and still am) for some kind of fire/water typed pokémon, but in the meantime, a water move that can cause a burn is pretty cool.
Aeroblast.
And its Gen II animation... just awesome.
That is cool. They had to be creative in gens I and II when it came to move animations. Though I really think gen II did a much better job with that than gen I, haha. Aeroblast looks really cool, in any case.
I have always been a fan of Ice Beam ever since using it with Articuno on my Blue version soooo yeah Ice Beam for me!
I can agree with Ice Beam! Not for the very best move, but certainly a candidate up there. Reliable and strong.
As for my favorite damaging move, it's always been Surf, right back to the original games. It has a pretty solid base power considering it's an HM, it has a cool battle animation in most gens, and it's arguably the most useful move outside of battle as well. Volt Tackle's another move I've liked using as of late- and probably one of my favorite moves used in the anime as well.
I love Surf. It's the only HM move you never regret or even hesitate teaching a pokémon because it's just that good so you wouldn't really need another water move. At least not in-game. At least not for me, haha. Even though Scald is cool, yeah.
In the earlier generations, when double battles wasn't a thing (mostly in Ruby, I'm thinking here) Earthquake was my favorite move. Still is, somewhat. It's so strong and cool and ground type that does critical damage to a lot of types. I generally like ground type moves actually, even though ground type pokémon aren't really my favorites.
Other than that, Slash is a given. Zangoose with Slash and Swords Dance - ain't nothing beating that in my Hoenn.