What do you mean by best? You mean a historigraphically and topically interesting piece on something specific? Or more like the broadest and clearest survey? Any preferences in terms of western/non-western history? Modern or antiquity? etc.
Shigehisa Kuriyama's
The Expressiveness of the Body is an interesting, rich and readable book, IMO.
Frontier Medicine by Dary is entertaining and readable, but not especially unique methodologically.
Foucault, while not exactly history of medicine, is always a good read for anyone thinking critically about medicine (
History of Sexuality or
Birth of the Clinic esp). Same with Scarry's
The Body in Pain and Sontag's
Illness as Metaphor/AIDS As Metaphor.
Charles Rosenberg's
The Cholera Years is a formative text in history of medicine. Also
Explaining Epidemics.
Other good ones:
No Magic Bullet (Allan Brandt)
Colonial Pathologies (Warwick Anderson)
Packard's Malaria book(s?)
Alan Kraut or Howard Markel if you want to read about immigration and the history of medicine.
OH! Also AIDS Doctors: Voices from the Epidemic is not exactly history, it's an ethnography of physicians working on the frontlines of AIDS early in the epidemic in the U.S., but given the passage of time and changes in HIV/AIDS care, it reads like a primary document. I highly recommend it as it's a poignant and eye-opening look at the medical professionals who put themselves on the line (often without any ability to gauge the risk they faced) to care for people who were pretty horribly marginalized by the rest of society. Anything by Cindy Patton, Paula Treichler on AIDS is solid, too.
Anyway, sorry for the entirely too long list but many of these are "standard reading" in history of medicine qualifying exams lists or upper level courses and make for good pleasure reading, too. If you were looking for suggestions, they might be a good place to start. If you just wanted a discussion on the actual best book then I have thrown this way off, in which case-apologies!