Easily chose Huge Power. Even with Belly Drum, Regular Aqua Jet and Knock Off won't nearly do enough damage to KO many things with decent defenses. However, with Huge Power, Azumarill finally reached the heights that brought it into OU. Basically a 400 base Attack Stat with Belly Drum with a Priority STAB move. Also, there are very few viable Pokémon in OU that have access to any Grass type moves to consider running Sap Sipper, especially on an offensive Azumarill. Smogon also says to run Huge Power.
Hope this helps!