Which sentence best translates They should forget it?

The best translation of "They should forget it" depends on the context and the desired level of formality. Here are a few options with different levels of formality and nuance:

Formal:

* They ought to forget about it.

* They would be well-advised to forget it.

Informal:

* They should just let it go.

* They need to forget about it.

* They should drop it.

More direct/blunt:

* They should forget it. (This is the most direct translation, but it can be a bit harsh depending on the context.)

The best choice depends on your specific situation. Consider who you're speaking to and what tone you want to convey.