nbme 3 or 4 are supposed to be the hardest so one would think that if you do well on those you'll do well on the real test. Nbme1 and 5 are supposed to be the easiest.
any of the NBMEs or the free 150 sample questions would be good for establishing a baseline. I didn't think that any of the NBMEs was more representative than the others. others might disagree.