Writing a recommendation letter for a student I reported for academic dishonesty
The best option is:  Don't write the letter.  In a case like this, I would outright refuse to provide a recommendation.

However, if you are not comfortable with a non-negotiable refusal, you might want to simply tell the student that you cannot provide a positive letter for them.  If the student insists, then you should write an honest letter that gives a balanced evaluation of their ability and personal traits.  The fact that you caught the student cheating is a critically important piece of information in this regard, and it needs to be included. The cheating will almost certainly be the most salient feature to anyone who reads the letter, which will make the net effect of the letter, regardless of whatever else you say in it, strongly negative.  And if the student insists on you writing the letter in this situation, that is basically what they deserve.