We have a Trimax UMAX100, a MasterLock 2847DAT latch lock, and a padlock connecting the safety chains behind the jack. The Trimax and latch lock are a bit loose due to the shape of the underslung coupler, but they lock up tight enough that you'd need serious tooling to remove them. The Trimax is all steel on the inside of the plastic cover, so it's pretty solid.
But...as everyone says these locks keep honest people honest. Anyone with a sawzall, angle grinder, hacksaw, crowbar, bolt cutters, big hammer and chisel could remove pretty much any coupler lock setup you choose. Some might take a bit longer, but they cost serious $$$ so you better be ready to pay.
If you want a real beast of a coupler lock and have $300...here you go, people seem happy with them:
https://www.provenlocks.com/products/model-2516-j
That model fits underslung couplers if that's what you have on your trailer, if not then they have other models. Also has a place to padlock your safety chains to. However I could still get that off if you gave me 10-15min and some good tools or a cutting torch. Still...that's about 10min longer than most of the other cheaper locks.
The cheap yellow Reese lock isn't wide enough to fit over the underslung coupler on our White Hawk.