It depends on the situation. If you are a borderline applicant, a research year can be huge in allowing to match into a program. If you want to match at a top program, a research year will only help so much.
I was a solid applicant (high Step 1 and 2 scores, AOA, several first authored papers), however, I attended a mid-tierish MD school. I thought an external research year would help me get invites from the top 10 programs; it didn't (well I did interview at a few, but not as many as I thought I would). While AOA and research output are important, I think the name of your school is just as if not more important that your step 1 score. I was amazed by the number of applicants from top-10 and top-20 schools with lower board scores and a fraction of my research output who were getting interviews at all the top programs.