What is the largest most powerful battery for commercial use?

One has to define "most powerful" but the largest to my knowledge is the BCI (Battery Council International) Group 8D. There may be larger special purpose batteries available but the 8D is the largest you're likely to find at any battery, marine, and truck supplier.

Even in very large storage situations like ships the practice is to use many 8D batteries. Anything larger is too difficult to be handled by a person as 8D batteries are approximately 140lbs.