Opinions on best anti theft hitch lock type device.

Carry insurance and if it makes you feel better put a $3 lock thru the latch, so it can't be opened.

If they really want it, you aren't going to stop them. At best you might discourage casual theft (tie a big orange ribbon around the $3 lock so they can see it) but any trailer worth stealing and you are up to Grand Theft charges, so not a casual action.