If you want to be very scientific about this, and to calculate cumulative sums of harm, with no discount for the future, then just look for some little-known hydrocarbons corp - it will top the list.
If you apply a future discount, but no discount (or a small one) for the suffering of non-human animals, then some meat company will probably top the list.
IMO most of the suggestions here are small beer.
If you want to be very scientific about this, and to calculate cumulative sums of harm, with no discount for the future, then just look for some little-known hydrocarbons corp - it will top the list.
If you apply a future discount, but no discount (or a small one) for the suffering of non-human animals, then some meat company will probably top the list.