This is just one factor of a letter, but I like when the letter writer gives examples of the applicant's qualities. It's one thing to say that an applicant works well with others, but if the writer goes beyond that and describes a situation where the applicant exemplified great teamwork or some other quality, that takes the LOR to the next level.
Also when the writer can give a sense of the relative strength of an applicant. Saying "Joe received an A in my class" is worthless. Saying "Out of the thousands of students I have taught in 25 years of teaching organic chemistry, Joe is among the top 10" is a lot more useful!
At this stage, pick people who know you well. It's nice to have a well known letter writer, but the reality is that most interviewers won't know the famous organic chemists or biologists you may have worked with. Even if you worked with someone well known in medicine, the surgeon/family doc/obgyn who interviews you will likely not recognize your GI letter writer. Pick people who will take the time to write a great letter and know you well enough for it to mean something.