Blastoise or Starmie?

IMO, Blastoise. Starmie has the higher Attack, but Blastoise will last longer in a fight.

But the real answer is Vaporeon...

You literally answered your own question perfectly.

if you based yourself a dodger specialist like dodge every single attack then starmie but you often dodge then blastois

Until gen3, not a single Water type can be the king of water but Vaporeon.

Best for what? Blastoise -- WG+HP can be great for prestiging... more often useful for prestiging than Vaporeon.

Vaporeon overall best because of its HP. But if you are facing a defender like Flareon or Rhydon I usually bring out my Starmie to take down the defender quickly. It is crucial to dodge charge moves though. Blastoise is irrelevant in my eyes.

Vaporeon. But out of the two, I'd say Starmie. Blastoise is only better than Vape in defense, but in my opinion, defense and stamina should always be looked at together. Starmie has a better attack stat, so there are conceivably situations where you might use him. I intend to eventually power up a Starmie, but have no plans to power up my Blastoise.

As many others alluded to, Vaporeon does what blastoise can do better. What starmie can do is resist psychic.

I use my Starmie at 1000-1200CP as one of my main Prestiger. Work every times, but you have to be a dodge-master.

Anyway, I don't have Blastoise at that range and at above 1500CP I have an army of Vap.