Senbonzakura is easily one of the best zanpaku-to shown in the series, as well as one of the cheapest. According to the official description, the katana itself breaks down into one thousand smaller blades, when reflected off of light look like cherry blossoms. Each of the 'petals' in return can be turned into a full-sized Senbonzakura, and be controlled as though they were a petal. Against most opponents, it seems such a thing is kind've unfair. In the history of the Bleach storyline, Byakuya has stated only Ichigo to have been able to parry the blades in their petal form.
The Ban Kai form is even more impressive, as the number soars to ridiculous amounts of petals. I'll finish this up soon.