It not just a filter that's important, but the whole system.Do you suggest a certain filter? I want the best
It'll take up almost as much space as your pond and might cost more.
That's for a bare bottomed pond with pristine water,
anything less will entail mucking out the pond every year to avoid algae blooms and other nasties.
Here is a very rough concept drawing of a good gravity fed filtration system:
The first, and most important, vessel is a settling tank to get all the heavy soil(poop) out of the water column.
All the vessels should have drains in them (not shown) to dump waste frequently from water column with the least amount of water replacement needed.