Hey, I just took the Step a month ago so I'll give you my experience. Make sure to do UWorld Self-Assessment 1 (UWSA1). For me, this was by far the most predictable (over predicted by 4 points). This is in contrast to NBME 17, which I took 2 days before the real deal, which underpredicted my score by 15 points (yes you read that right).
I did NBMEs 13, 15, 16, 17, and UWSA 1. The most predictive for me personally were 13 and UWSA 1. 15 was my lowest (22 points underpredicted).