I would go for Drain Punch for a couple reasons.
A) You have two already powerful special attacks.
B) It hits Dark types really hard which is nice for dealing with Ghetsis's Hydreigon, Grimsley, and Iris's Hydreigon.
For that fourth move I suggest Aqua Tail which allows you to hit ground types.
With the four moves, Aqua Tail, Drain Punch, Flamethrower, and Thunderbolt, the only two Pokemon you aren't hitting are Giratina and the Lati twins, which you won't be seeing used in-game by anyone other then you.