In cover letters is it best to use past tense or present perfect tense?
Present tense best describes current and potential employment in a cover letter. Because present tense is here-and-now it makes your cover letter more relevant to immediate job openings.
If you discuss previous work experiences in your cover letter to support job-related qualifications, use the past tense. Otherwise, a hiring manager might think you're still performing those tasks and wonder if you're immediately available for hire.
Avoid passive voice and maintain consistency with your present and past verb tenses throughout your cover letter.