Kingdra'a moveset

I have a Dragon Breath/Hydro Pump, and I personally recommend that one because you can use it more easily against foes like Dragonite and Exeguttor in a pinch.

Low CP means kingdra is mostly a prestiger and you will have to dodge a lot.

Kingdra has double resistance to fire and water. So her ideal moveset is something that counters both fire & water.

DB/O works pretty good against fire and water
WG/HP works extreme well against fire but you will struggle when you face high health water types.

DB/HP is better but DB/O is best by a slim margin.

Firstly, I would not use any stardust on it. It is strictly a prestige, and should stay at their current CP. As far as most useful, it depends on what you are attacking. Outrage is great for dragonites, but remember Kingdra is dragon type too, just all dragon type attacks will benefit from STAB. Kingdra's HP is really low, so it can't take that many hits. I used my DB/O and was not very impressed. My 1300 IS/SE Piloswine is better.

DB/HP can be used against rhydon, donphan, snorlax, blissey, golem. But you probably already have better water prestigers. I use my starmies and golducks all the time.

To counter dragon you wouldn't want any water moves.