This question is something I am wondering about too although I will share with you some advice I was given.
Fellowships. Look at 2 things with each program: 1) what kind of fellowships is the residency program affiliated with? and 2) how many of their graduates go into fellowships and where?
From my understanding, a competitive program is a good academic program. A good academic program is one which is affiliated with several fellowships and places its graduates into those affiliated fellowships.
Another criteria to consider is research. Are residents required to do research? How much research? What are some research projects going on at the program right now? How many from the faculty are actively involved in research?
One more criteria I was told to consider is look at the current resident class and see where they went to medical school. This will give you some idea about the competitiveness of the program since competitive program will have residents from "top" medical schools.
Again, like I said before these are things I have read or have been told. I'm not sure how much of it applies or is true.