Prestiger for Gyarados?

To me by far the best option because of its high HP. I cannot praise Lanturn enough. My 1600 CP Lanturn takes out around 2 Gyarados before it faints. It's ok w/ Vaporeon, but it's way better w/ Gyarados.

As mentioned above, Lanturn is pretty good when it comes to bulk and potion efficiency.
But if you're not worried about revives/ potions, any Pokemon with 2 electric moves will do... Gyarados isn't the tankiest mon and his dual weakness is easly exploitable.

On paper he seems really good with CB/TB. CB having a 1.1 second cd is a bummer for me though. TS or Spark would have been awesome but then he probs woulda been too OP against Gyara and Vape.

For me 1300 cp wigglytuff can take on 2 2800 cp gyraddos only dodging specials. I cant find any other use for raichu though so use them quite a bit since they are fun to dodge with and they come with different hats :)

IMO the best moveset for electric is TS/TB or T so id go with mons that have that.