I don't think this document specifies the average class rank, but it has been awhile since I last read the whole thing. It does tell you for each specialty, about general characteristics of people that matched: # of research experiences, % who has AOA status, # of work experiences, average step 1 score, step 2 status, number of programs ranked, etc.
http://www.aamc.org/matchoutcomes
Edit: Far as grades go. I'm not sure how programs treat the basic science years. But my impression from talking to department chairs is that 3rd year is by far the most important year. Of course with emphasis on ob/gyn, and possibly surgery and internal medicine. In my school, you GPA is calculated with your 1st year, 2nd year, then 3rd year counts as two years. Maybe your school has something similar?
However, I advise keeping an open mind
😉. I would try to do well on everything and especially 3rd year. I kind of blew off my grades assuming I would do a very noncompetitive specialty. I feared ob/gyn and now I'm applying for residency. It certainly would have been helpful in getting a more competitive spot (now i'm apply everywhere xD).