For me...Aqua Tail.
I hit a gym that had 6 Rhydon in a row...so I took the chance to test exactly this issue with Aquatail, Water Pulse and Hydro Pump Vapes in to do the damage.
Hydro Pump is awesome and gives a massive hit, but has limited options - you basically have to accept you will get hit, or at least wait a long time to use it.
The pain inflicted is worth taking a hit for...much power.
Water Pulse will mean you are probably going to take a hit, but it isn't going to be a massive damage - it was handy to start the next fight with a charge bar, if timing is good.
Not enough damage done to warrant taking the hit (at least HP only takes one hit per 100 energy)
Aqua Tail - my personal fav from the quick test (I may change my mind). It feels like you have a charge available all the time, and can even get hits off between dodges. It grinds down the opponent, but it does it fast and very efficiently...at least if felt like that.
I may change my mind, as I get more used to all the moves - but from a first test Aqua Tail felt best as an attacker.
Your mileage may vary.
