I don't think it's the very highest, but the Sloshing Machines deserve a place in the poll, at least. Unlike other buckets, they are not guaranteed two-shot splats; the high-damage epicentre of the attack is really difficult to hit a dodgy opponent with, due to the start-up lag of every attack, and if you don't get a direct hit in the first two attacks, it'll take a third, assuming all three at least strike indirectly. And even with just two hits, this weapon has one of the most pathetic TTKs, if not the worst, of anything that doesn't need charging up.
On top of that, aside from the Inkbrush and possibly regular Rapid Blasters, SMs have a lower range than anything listed in the poll. Aaand, although the shots work like those of a (not so) Rapid Blaster, there's no explosion in the air, and a shot is completely nullified if the epicentre hits a solid surface (even if it seems like the fringe of the shot should carry on a little farther).
I make the worst sales pitch ever, but, honestly, I adore these weapons, which is why I draw attention to them. What they have got going for them is that they can do a few things that, very literally, no other weapon can do. For instance, although, in theory, all buckets are meant to be capable of attacking over walls or up/down ledges, the shots of the Tri and Slosher are very prone to being absorbed by such surfaces, whereas one can get the Machine's epicentre past such obstacles with more versatility. It's also able to attack over Splash Walls, although one needs a little cover of one's own and actually aiming at the opponent becomes a whole new mini-game, as you have to fire upwards to do so...