Don’t exaggerate -- show where and how they can improve. Don’t put the candidate on a pedestal. Not only does it not look plausible, it will also set expectations for them that will be next to impossible to meet. If they have an Achille’s heel, don’t exaggerate it, but don’t hide it, either.
Target the recommendation. Is it for an academic post, a job, a volunteer position, a background check, or a personal reference? Write your letter so that it is focused on this purpose. For example, if the letter is part of a job application package, it should focus on the professional qualifications and conduct of the candidate.