Currently working on my team, and already finished doing the EV to three of them, but seriously, I don't know what to give to Gyarados (Magikarp at the moment)

I was thinking of boosting it's attack to the max, 255.. maybe Speed to attack faster?

One question though: I have this moveset:

Earthquake, Torment, Flail, Dragon Dance.

It learns Flail on lvl 30. by being Magikarp, but the thing can evolve on level 20. Is it worth it to keep it Magikarp till lvl 30. to obtain this move? Thanks!