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Water spefically? Team Up Blastoise followed by Frosmoth.
Emboar from Cosmic Eclipse is a distant 3rd because its ability triggers only when you evolve into it, but it goes 8 cards deep and does not specify a specific type, but if you're thinking seriously about Emboar you're better off running Blastoise.
Basically, anything that helps with consistency for Frosmoth or Blastoise is what you'll be looking at, assuming you mean Standard.
Also iirc the best acceleration pre-rotation was still Naganadel and Quagsire combo. Turn 1 is a very big ask as Pokemon's design team has been shying away from such strong acceleration, though that was also quite uncommon to begin with.