What is the worlds largest ferret?

There's no such thing as a "world's largest ferret". Ferrets are a specific species, and while some individuals might be slightly larger than others, there's no significant variation in size that would make one a "world's largest".

What you might be thinking of is a giant ferret, which is a mythical creature. It's a fictional creature, not a real animal.

If you're interested in the largest weasel-like creatures, then you'd be looking at animals like the Wolverine, or the Giant Otter. Both of these animals are significantly larger than the average ferret.