Nbme 12
Of the 6 NBMEs that are currently available online officially, form 12 definitely felt the hardest to me.
Had a lot of vague questions and felt like I was guessing a lot of the time. Its also the NBME I scored the lowest on.
NBME 17 felt the most straightforward, but the curve is pretty steep, hence not as scoring.
NBME 18 was the kindest to me in terms of 3-digit score.