The Rainmaker itself.
No, well, the Tri-Slosher, I guess, although there are exceptions on a couple of stages. This weapon is wonderful for taking out the enemy carrier as it's easy to hit them once, after which they'll normally be unable to escape your ink before you get the second slosh.
Bubbler is also good for bringing together a group defence, worrying the opponents or, particularly, insuring one-on-one situations wherein one normally would be disadvantaged. The nature of this mode leads to scattered opponents a fair amount of the time compared to other Ranked battles, which is generally advantageous to buckets, as they have a huge TTK relative to their range and so really struggle against groups.
However, my one absolute favourite pastime in this mode has got to be, when it looks like our team is in a bad spot as they have the Rainmaker and a numerical advantage, finding a safe and somewhat distant spot and Disrupting the enemy carrier. My philosophy here is that it doesn't matter how clear a path the enemy has to the goal if their Rainmaker can't move.
... but back to my original point. I'm only half-joking when I say that I love running the Rainmaker. I find that, whereas in the other two modes, there's a limit to how much one can carry one's team and one is therefore always at the mercy of the matchmaker, RM allows one to make single-handed plays that completely change the flow of the game just by grabbing the cannon and slipping through a fatal crack in the enemy line. Also, once you have the Rainmaker, if your original weapon happened to be seriously troubled by the opposing team this round, it no longer matters - of all four members of your team, you probably improved more by grabbing the Rainmaker than any of the others. Of course, one still needs to have restraint and try to hold out until the best possible opportunity for a run arises before grabbing the item or before advancing with it. But my point is that, much of the time, anyone on the team is a valid carrier, and I just prefer to be that person myself.