What is the longest-range torpedo hit in history?

That's a harder question to answer than you might think. That it's a Japanese torpedo is pretty well accepted. But there are two competing ships here. The cruiser Haguro, at the Battle of the Java Sea being one. The other being IJN Niizuki, a destroyer in the Solomons. That is more commonly brought up these days, but exactly how far she launched her torpedoes is...disputed.

What isn't disputed, is the fact that she used radar to launch those torpedoes. It's rare when the Japanese get the better of the Americans with radar at all, and Niizuki is the golden example there.

Further Reading:
https://www.history.navy.mil/research...
http://www.combinedfleet.com/niizuk_t...
http://www.combinedfleet.com/niizuk_o...