I took 17, 16, and 15 respectively. I found 17 easier that 16 or 15, so my jump in score was lower that expected when moving form 17 to 16.
The later the NBME was released, the better it is to take IMO, but your school may not have data on the newest ones to correlate with performances of past peers.
At my school, NBMEs were shown to be a great measure to track your progress, but the only metric that was directly correlated with higher USMLE scores was number of Qbank questions completed and reviewed.