In the Mass area, it would be MGH and Brigham and Women's Hospital.
You should think about what qualities make a hospital "best" in your mind and if those qualities fit with the work atmosphere that you want for your career and life. For example, some hospitals are teaching hospitals and consequently there are many nursing students, medical students, medical residents, and medical fellows around in addition to experienced staff. These hospitals also tend to do/have a lot of research, which may translate into more cutting edge approaches. Some hospitals value incorporating holistic and/or complementary therapies. Other hospitals may have a spiritual or religious component worked into the philosophy and fabric of the hospital.
That being said, a widely referenced guide to hospital care is http://health.usnews.com/best-hospitals/rankings They do an annual report. You can also research through some of the accrediting bodies such as JCAHO: Joint Commission on Accreditation of Healthcare Organizations.</body></html>