I have already Vaporeon, I needed it to complete the first history of the game, now I want to win all the towers of the Battle Frontier, but I think that there may be other Pokemons with better perfomance. I am thinking on Gyarados and Milotic, but I'm still in doubt which of the three would be the best to go into frontier battles. Which do you think that is supposed to be the best water Pokemon for frontier battles? Remember: I already have Vaporeon.

Vaporeon (bulky water)
Nature: Modest (+S Atk, - Atk)
EV's: 252 HP, 252 Sp Atk, 6 Sp Def
Ability: Water Absorb (absorb water moves and converts into HP)
-Ice Beam

The fact Vaporeon got A LOT of HP allows it to stand in field and count on Toxic to defeat most of the enemies. His ability is most of the times useless as no opponent will use a water move at it (maybe on a double battle as Surf and Muddy Water are able to hit all Pokemons in field). Should I replace Dive with Surf or Hydro Pump? I taught Dive because I gain one extra turn to Leftovers heals HP and foe gets damage from poison (which is doubled on the next turn).

Milotic (special sweeper)
Nature: Modest (+S Atk, - Atk)
EV's: 252 Sp Atk, 252 Sp Defense, 6 HP
Ability: Marvel Scale (double defense if gets a status condition)
-Ice Beam
-Dragon Pulse
-Mirror Coat

Much a coincidence with Cynthia's Milotic's moveset, but it can't be built any better (I could replace Surf and Ice Beam with Hydro Pump and Blizzard, but their Accuracy is a risk). No much to say about this Milotic. It was going to be a special sweeper. To resist to this Milotic it was needed a special wall like Blissey, or a high special-defensive Pokemon like Snorlax.

Gyarados (physical sweeper)
@Focus Sash/Shell Bell
Nature: Adamant (+Atk, -S Atk)
EV's: 252 Atk, 252 Sp Defense, 6 Spd
Ability: Intimidate (lowers foe's defense when switched out)
-Swords Dance
-Aqua Tail
-Ice Fang

I have to use Swords Dance first than any other move unless that one of Gyarados' three damage moves matches up well against foe's type. In case foe got an electric move, Focus Sash will prevent a instant knock out when Gyarados' HP were fully restored. Another suggestion as hold item could be Shell Bell as Gyarados usually does massive damage.