I was looking for their types and found this. So I'll just go ahead and make my own assessment.
I'm currently in mid of season 5, so I type them accordingly.
Mark Greene
I connect to him very strongly, so I'll go with ISTJ. Always ready to pull insane hours, puts duty first but takes no prestige in his accomplishments. When facing injustice, expresses anger. Can easily be mistaken for to be boring.
Doug Ross
ENFP. Many ways the opposite of Mark. Charming and easy-going, but not dependable. Very idealistic and has problems with authority and structure. Also has difficulties carrying responsiblities and long-term relationships.
Susan Lewis
Could be ESFJ. They had a common understanding with Mark, even though they failed to communicate it. Finds it hard to be assertive and doesn't like to make noise of herself. She also kinda expects people around her to also behave. Is irritated when her sister gets to be mother, althought she's not very dedicated to it.
Carol Hathaway
ISFP. Is good at comforting others, but tends to take guilt over things out of her control. Tends to be bit naive, althought tends to fight back.
John Carter
ENFJ. He's pretty much interested in everything and everyone.
Peter Benton
INTP. Mostly clueless about social interactions. Is very concentrated on what he's good at. Finds it very difficult to discuss subjective and emotional topics.
Jeanie Boulet
ISFJ. Quiet and reserved, but caring and considering. Is not assertive at all and respects people around her highly.
Kerry Weaver
ESTJ. Has a need to organize people around her and is almost compulsed with efficiency. Can be bit harsh in her assessment, but generally correct. Mostly does not hold grudges, but dislikes rule-breakers.
Elizabeth Corday
ESFP. Does not mind blending in and out of any situtation.
Robert Romano
ESTP. Goes for dominance and is as direct as one can get. Althought might seem like uncaring and clueless, he has very keen sense of people's thoughts. A very much a player.