Babylon 5 (cosmology) - Dr. Stephen Franklin and Dr. Sarah Chambers.
Do you really mean cosmology? Anyway I found Dr. Stephen Franklin in Wikipedia, but not Dr. Sarah Chambers. So I'm adding the former.
seaQuest - Dr. Kristin Westphalen, Dr.Wendy Smith, and Dr. Perry.
Of these, the first seems to be the "most important" one, and I would add her to the list. What do you think about it?
ER: (The following appeared in at least 6 episodes.)
Oh no! I know in advance there would be far too many of them from that series. Just name the one which in your seemingly far from being humble opinion is the one there.
Not sure, this poll proposal, which initially seemed interesting is now slowly turning somewhat into a nightmare and I find it unlikely to be accepted anyway. Perhaps I should have better stated the scope of "Dr." charachters to be listed as options...